Faster access to PPField.lbproc

Recent tests of the fast-FieldsFile code have shown that a substantial fraction of the total file-to-Cube conversion process is taken up constructing instances of `BitwiseInt` for `field.lbproc`.

This work-in-progress PR demonstrates an alternative, compute-on-demand implementation which gets rid of the `BitwiseInt` overhead in the fast-FieldsFile case. Comments welcome.
How about I ...
 - deprecate `BitwiseInt` completely,
 - deprecate the inheritance between LBProc and SplittableInt (by overriding the subclass check to warn if someone's doing `isinstance(my_lbproc, SplittableInt)` or `isinstance(my_lbproc, BitwiseInt)`.

(Assuming the performance impact is negligible.)