Pp stash constrain

Bugfix for problem introduced by pp constraint 'conversion' code in v1.7.
Various users had code with the idiom ```constraint = AttributeConstraint(STASH=STASH(x,y,z))```.
When used to load, 'pp._convert_constraints' refused to handle these, as it expected the test value to be a string (only).  Fixed that by supporting a STASH object in the convert code.
NOTE: there were problems here with the message formatting in the error handling code:
```'%r' % <STASH-object>``` does not work, as the object is a tuple, which it tries to use as an argument list.