PP constraint

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

1. It's bad form to use a mutable value, such as `{}`, as a default parameter value. Just `None` will do fine.
1. The overloaded use of `constraints` is confusing terminology - at this point this is just a dictionary of desired attributes.