Added methods to check if below sea-level points were considered sea …

[MichaelClerx]

…when the mask was generated and the sea slope was added.

@EricWay1024 this adds a method is_sea that accepts coordinates and tells you whether nevis thinks it's sea or not

Also a method inland_below_sea_level that returns coordinates for all grid squares below 0 but not considered sea.
This returns points in meters, but you can divide by 50 to get matrix indices.
Alternatively, we can expose the hidden method inland_below_sea_level_indices to return those directly.

[EricWay1024]

LGTM 👍