Mpxj

* The project property Critical Slack Limit is now represented as a `Duration` rather than as an `Integer`. (Potentially breaking change if you were using this property directly).
* `TaskType` is now a simple enum with all Microsoft Project specific functionality moved into `TaskTypeHelper`. (Potentially breaking change if you were using the `TaskType` methods `getInstance` or `getValue` in your code)
* When reading the task type from P6 schedule the mapping to the MPXJ `TaskType` enum has been updated to more closely match P6. The main changes are that the P6 type "Fixed Units" now maps to `TaskType.FIXED_WORK` and the "Fixed Duration & Units" type now maps to a new enumeration value `TaskType.FIXED_DURATION_AND_UNITS`.
* Added support for reading project calendar exceptions from Phoenix schedules (based on a contribution by Rohit Sinha).
* The Resource attribute Active now defaults to true if the schedule being read doesn't support or contain a value for this attribute.
* Add support for reading and writing the Resource's Active flag for P6 schedules.
* Add support for reading and writing the Resource's Default Units/Time value for P6 schedules.
* Add support for reading and writing the Project's Critical Slack Limit value for P6 schedules.
* Fixed an issue reading certain types of Enterprise Custom Fields containing date values.
* Ensure activity code value parent can be set to null.
* Improved existing .Net extension methods and added support for more types.
* Added NuGet package icon
* Simplified NuGet packaging

* Write activity code definitions and activity code assignments to PMXML files.
* Added support for "secure" and "max length" attributes to the `ActivityCode` class.
* Added `getChildCodes` method to `ActivityCode` and `ActivityCodeValue` to make it easier to traverse activity code values hierarchically.
* Added `setDescription` method to `Step` class to make it simpler to add a plan text description.

* User defined fields read from P6, Asta and GanttProject schedules are now represented by instances of `UserDefinedField`. They will no longer be mapped to custom field instances.
* Enterprise Custom Fields read from MPP and MSPDI files are now represented by instances of `UserDefinedField`.
* When writing MSPDI files, UserDefinedField instances which were originally read from enterprise custom fields will be written to the MSPDI file as enterprise custom fields.
* When writing MSPDI files, UserDefinedField instances which were from applications other than Microsoft Project will automatically be mapped to available custom fields.
* When writing MPX files, UserDefinedField instances will automatically be mapped to available custom fields.
* The `UserDefinedField` type implements the `FieldType` interface and so can be used with the `FieldContainer` `get` and `set` methods to work with the contents of the user defined fields.
* The `ProjectFile.getUserDefinedFields()` method has been added to provide access to all user defined fields defined in the project.
* The `CustomFieldContainer` returned by `ProjectFile.getCustomFields()` will contain entries for all `UserDefinedField` instances.
* The various `getFieldTypeByAlias` and `getFieldByAlias` methods will retrieve user defined fields by name.
* Added the convenience method `ProjectFile.getPopulatedFields()` to retrieve details of all populated fields across the project. This avoids the caller having to individually retrieve the populated fields from the tasks container, resource container and so on.
* Updated the `getPopulatedFields` methods to return a `Set` of `FieldType` rather than a `Set` of `TaskField`, `ResourceField` etc.
* The various `getPopulatedFields` methods will include instances of `UserDefinedField` in the returned collection if relevant.
* All `ENTERPRISE_CUSTOM_FIELDn` values have been removed from the `TaskField`, `ResourceField`, `AssignmentField` and `ProjectField` enumerations.
* The `getEnterpriseCustomField` and `setEnterpriseCustomField` methods have been removed from `ProjectProperties`, Task`, `Resource` and `ResourceAssignment`.
* Project UDFs are now read from P6 schedules.
* Project UDFs are now written to PMXML files.
* All code previously marked as deprecated has been removed.

* Updated to improve reading resource attributes from certain MPP14 files.

* Updated to make resource curve definitions (work contours) available in the `WorkContourContainer`. This container is accessed using the `ProjectFile.getWorkContours()` method.

* Improve accuracy when normalising timephased data.
* Add support for reading activity steps from XER files, PMXML files and Primavera databases.
* Add support for writing activity steps to PMXML files.
* Updated PMXML schema to version 22.12.
* Updated methods in the `GanttBarCommonStyle` and `GanttBarStyle` classes to use a `FieldType` instance rather than a `TaskField` instance to allow more flexibility. (Note: this may be a breaking change if you are currently using these classes.)
* Optionally include some Microsoft Project layout data in JSON output.