#77 Massive refactor + supporting CSV-W HTTP Headers Properly

[robons]

• Moved from using Jenkins to GitHub Actions for PR + Master builds (were already using it for releases).
• Refactored code to use the Either class instead of exceptions - this helps ensure we're more robustly covering all edge cases -- I've put a lot more Metadata error messages in to the code base as a result.
• Now mocking HTTP requests from the cucumber tests so that we can test the HTTP header support. (Tangentially achieved #77).
• Split up the up some of the PropertyChecker class into the smaller Normalisation classes to make the code base a bit more maintainable/comprehensible.
• Removed the unhelpful partitioning of properties in TableGroup/Table/Column. Property inheritance is done is a more transparent fashion now.
• Fixed a bunch of validation which wasn't doing exactly what it should have been doing.

The only thing I can think it would be worthwhile removing now is the unnecessary PropertyType enum and the necessity to pass it in to each of the normalisers - but it doesn't cause toooo much pain.

This PR doesn't leave the project in any kind of perfect position, but I'm confident that it meets 98+% of the W3C's CSV-W specification. I believe it leaves the code-base in a state which is much closer to maintainable than when I found it - but I do want to emphasise that it falls a lot short of what I would like to have done, had I more time.

[github-actions[bot]]

Unit and Cucumber Test Results

482 tests   482 :heavy_check_mark:  29s :stopwatch:   17 suites      0 :zzz:   17 files        0 :x:

Results for commit 8b480788.

:recycle: This comment has been updated with latest results.