Fix key names when loading GDT12
Merge pull request #1486 from rhattersley/fix-gdt12-keys

Fix key names when loading GDT12
allow calculation of forecast period from forecast ref time
More tolerant coordinate loading for GDT12.
Merge pull request #1487 from esc24/calc_fp

allow calculation of forecast period from forecast ref time
Fix X/Y unit handling for GDT12 export.
Merge pull request #1488 from rhattersley/tolerant-gdt12

More tolerant coordinate loading for GDT12.
Merge pull request #1490 from rhattersley/gdt12-export-unit-handling

Fix X/Y unit handling for GDT12 export.