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
Fix typo and tweak test name
Merge pull request #844 from rhattersley/ff-no-coords

Fallback to using REAL_CONSTANTS.