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``` will refuse to handle these, as it expects the test value to be a string (only).  Fixed that by supporting a STASH object in the convert code.