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.
> LBPROC is "just an integer"

An excellent suggestion! :+1: 

Surprisingly, the current rules don't even make use of the `.flag256` style of access! They mostly just make comparisons like `f.lbproc == 8192`. The exception is to use `f.lbproc.flags` to create fall-back processing descriptions, but as you imply, we could now do this in the convert function.

So... your "just an integer" suggestion seems to be the best target to aim for in the medium-term (i.e. Iris v2), but right now there's the issue of backwards compatibility. My current `LBProc` class is designed to preserve compatibility (so allow it to go into v1.8) whilst giving most of the performance of "just an integer".