Check for offshore and test

[ximenesuk]

This PR adds the additional check for whether a location is completely outside both the UK EEA Offshore area and Northern Ireland landmass. Locations outside of this area will currently report two error messages:

{'desc': 'NATE / NATN outside UK Offshore EEA or Northern Ireland (Paris)', 'group': 'LOCA', 'line': '16'},
{'desc': 'NATE / NATN outside Great Britain or Northern Ireland landmass (Paris)', 'group': 'LOCA', 'line': '16'},

It would be possible to drop this to the single "outside UK Offshore EEA" error if that was preferred.

Locations within the offshore area but not on the UK landmass will report a single error:

{'desc': 'NATE / NATN outside Great Britain or Northern Ireland landmass (MorayFirth)', 'group': 'LOCA', 'line': '15'},

The unit tests should pass.

[KoalaGeo]

@ximenesuk looks good & works for me.

If we can slightly ammend the error message think it's good to merge

[ximenesuk]

@ximenesuk looks good & works for me.

If we can slightly ammend the error message think it's good to merge

Messages amended.

[KoalaGeo]

Addresses https://github.com/BritishGeologicalSurvey/pyagsapi/issues/102