Added land compressed PP/FF file loading.

This PR implements Unified Model land/sea mask de-compression for Fieldsfiles and PP files.

The functional changes themselves are moderate in complexity, but it is the testing of these changes (and making sure there is a suitable framework to build upon for future changes) which make up most of this diff.

Pinging @cpelley and @rhattersley, as I know they are keen to see this merged in the next ~5 days.