PP constraint

Speed up for PP and FieldsFiles where a constraint:
```
constraint = iris.AttributeConstraint(STASH='mXXsXXiXXX')
```
is used.

This is overly complex for what is actually supported.

Given the unknown future behaviour of this pre-filter idea I'd suggest encapsulating it to avoid it being split between this function and the `convert_constraints` function. The simplest encapsulation might just be a function.

For example:
```python
def load_cubes(filenames, user_callback, loader, filter_function=None):
    # ...
    if filter_function is not None and not filter_function(field):
        continue
    # ...
```

NB. You don't need to use `cube = None` to signal an irrelevant cube, a simple `continue` statement is fine.