Fallback to REAL constants where necessary.

For FieldsFile fields with no BDX, BDY and no X, Y vectors,
use the values in the file's REAL_CONSTANTS section instead.
Add abstract definitions
Comment and test name tweaks
Push regular_x/y methods to superclass
Revert handling of horiz_grid_type
name to grib
review actions
Fix typo and tweak test name
Use ubuntugis stable.
Merge pull request #857 from bjlittle/travis_gdal_fix

Switch to ubuntugis stable.
Merge pull request #844 from rhattersley/ff-no-coords

Fallback to using REAL_CONSTANTS.
Merge pull request #835 from bblay/name_to_grib

name to grib
Merge pull request #852 from bjlittle/refactor_grib_cf_map

Cosmetic reformat of GRIB/CF map.
Reorder GRIB/CF map entries.