Exetera

* BugFix: DataStore.get_spans returning None when passed Readers in legacy scripts. Functionality has been restored from v0.4

* Feature: DataFrame.rename function added; allows renaming of one or more fields within a dataframe

* Major changes to API
* Datasets & DataFrames introduced
* Rich API on Fields introduced
* Much functionality previously accessed through Session can now be accessed through Datasets, DataFrames and Fields
* See ([Basic Examples](https://github.com/KCL-BMEIS/ExeTera/wiki/Basic-Examples) and [Intermediate Examples](https://github.com/KCL-BMEIS/ExeTera/wiki/Intermediate-Examples) for more details
* Import improvements
* You can now specify include and exclude lists for fields in a table during import
* This allows you to improve import performance and dataset size by excluding or only including the fields that you are interested in

* Separation of all covid-specific functionality out to [https://github.com/KCL-BMEIS/ExeTeraCovid.git](https://github.com/KCL-BMEIS/ExeTeraCovid.git)
* Removal of legacy csv pipeline code
* Renaming of some of the `ordered_merge_*` functionality parameters for clarity
* Addition of `open/close/list/get_dataset` functionality to `Session`
* Made `Session` 'withable'
* Improved performance of `Session.get_spans`
* Bug fixes for Session API
* apply_spans / aggregation issues
* Bug fixes for Field API
* provided `__bool__` so that `if field:` works as expected
* provided single element read for `IndexedStringField`

* Fixing issues with use of test_type_from_mechanism_v1
* Adding ability to optionally import lsoa-based fields through add_imd script
* Import now appends by default; to overwrite an existing dataset use `-w` \ `--overwrite`
* Moved schema files to resources
* Adding separate lsoa schema for import

* Major performance improvement to Session.get_spans