Nullable time column

[na9da]

What this PR does

Makes timeColumn in table styles nullable. Useful for turning off auto-detection of time columns in tables.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated CHANGES.md with what I changed.

[tephenavies]

This looks good. Can you also add a test that all the correct entities are created in the dataSource when using null timeColumn and a timeseries CSV? Or at least the right number of entities. Maybe with this test file: https://github.com/TerriaJS/terriajs/blob/next/wwwroot/test/csv/lat_lon_timeseries.csv

There should be 11 entities created, one for each row (though they'll all be in 2 locations)

[nf-s]

AREMI Power generation test URL http://ci.terria.io/nullable-time-column/#clean&https://gist.githubusercontent.com/nf-s/9ff1422d9528c16a9e71403ec720558a/raw/fe86f1c9bf2dd4053148475dd1361fc7e205ec22/aremi-elec-infra.json

Done