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.
You might want to consider sticking `NUMBITS` into the `class_dict` in the metaclass `__new__` e.g.:
```python
class_dict['NUMBITS'] = cls.NUMBITS
```
so it becomes a class variable of `LBProc` avoiding the need for a literal.