Fix string formatting bugs in STASH object error messages.
Add pp constraint conversion test where STASH=stash-object (instead of string).
Allow STASH-constraint conversion where reference is a STASH object (instead of a string).
Merge pull request #1306 from pp-mo/pp_stash_constrain

Pp stash constrain
Merge pull request #1320 from SciTools/v1.7.x

Merge from 1.7