Added functions ex-SEEGsdm for dealing with point data and mask incompatibility

[dhduncan]

I added a couple of functions adapted from SEEGsdm to deal with occurrence points that may not coincide with a crucial mask or extent.

• inside_mask - to subset points to those that can extract from mask
• assign_nearest_land - to move points to nearest land (mask) within a specified distance tolerance

Also, a minor edit to raster_to_terra xref table to note behaviour change.