Rename linear interpolator sub-package.
Move snapshot_grid and get_xy_dim_coords.
Remove circular dependency.

 - Move extend_circular_[coord|coord_and_data|data]
 - Duplicate _LINEAR_EXTRAPOLATION_MODES.
GRIB2 GDT 4.
Review actions
Rebase and review actions
Merge pull request #1358 from bjlittle/grib2-gdt-4

GRIB2 GDT 4.
Merge pull request #1379 from rhattersley/tidy-interp

Break regrid circular dependency
GRIB2 grid definition template 5
Merge pull request #1380 from bjlittle/grib2-gdt-5

GRIB2 grid definition template 5
Support grib product definition template 4.9 (probability forecast).