PP constraint

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

Looks like there's another logic issue here. Supplying a known constraint and an unknown constraint will currently result in *only* fields with the matching STASH code passing the pre-filter, thus excluding fields which might match the unknown constraint.

Instead, whenever an unknown constraint is supplied the result of this function should be "allow everything".