Promote ABF/ABL and fix fill value and comparison.

1. Even though the ABF/ABL data type is an 8-bit unsigned integer, the default fill value assigned by np.ma.MaskedArray() is 999999! As a result, doing `cube.data.filled()` would substitute 63 for masked values!
2. Fixed bug in FormatSpecification comparison.
3. Promoted the loader from experimental.