Adds additional common netCDF coordinates

[JamesVarndell]

Adds a few more common geographic coordinates found frequently in netCDF data - namely:

• "X" and "Y" (capital X and Y)
• "xc" and "yc"

These additional geographic coordinates make it easier to work with datasets which use these coordinate names. These additional coordinates have been selected based on their use within some CDS datasets.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.78%. Comparing base (d5c6309) to head (6284f53).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #398 +/- ## ======================================== Coverage 89.78% 89.78% ======================================== Files 120 120 Lines 7653 7653 Branches 675 675 ======================================== Hits 6871 6871 Misses 651 651 Partials 131 131 ```

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