Xerparser

* Added properties to `PROJWBS` class:
* `project` gives PROJWBS objects access to the project and its properties.
* `original_duration` calendar days between start and finish dates.
* `actual_duration` calendar days between the project data date the wbs start date.
* `remaining_duration` calendar days between the wbd finish date and the project data date.

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* Added `all_task` property to `PROJWBS` class, which returns all tasks assigned to a node and all it's children nodes.
* Added `actual_cost`, `budgeted_cost`, `remaining_cost`, and `this_period_cost` properties to `PROJWBS` class. These are sums of the costs for all activities under a WBS node.
* Added `start` and `finish` properties to `PROJWBS` class.
* `PROJWBS` objects are now sorted by seq_num to match the order reflected in P6.

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* `Node` object now use `seq_num` attribute for comparison and sorting.
* Fixed potential bug in how children are stored in the `Node` class; switched from a list to a dict to avoid having multiple of the same child.

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NOTE: If you have tests setup, this change will require you to delete the existing `xer_data.json` fixture and recreate it in the test process.

* Added functionality for traversing `Node` objects:
* `traverse_parents` method iterates through parents to root node.
* `traverse_children` method iterates through all children to leaves.
* `height` property is the length of the longest downward path to a leaf from a given node (leaf nodes with have a height of 0).
* `depth` property is the length of the path to the root node from a given node (root node will have a depth of 0).

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* Fixed potential KeyErrors in the `SCHEDOPTIONS` class due to missing attributes in files exported from older versions of P6. [Issue 7](https://github.com/jjCode01/xerparser/issues/7)
* Fixed bug where `create_date` and `update_date` attributes of the `TASK` class may be empty. [Issue 8](https://github.com/jjCode01/xerparser/issues/8)

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* Added `tasks` (list[`TASK`]) attribute to `PROJWBS` class.
* Added `task_rsrcs` (list[`TASKRSRC`]) attribute to `RSRC` class.

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