imageset improvments:

    1) canonicalize transformation from the input
       to be Lambda
    2) if _eval_imageset wasn't able to compute
       image - we return an unevaluated ImageSet object
Add imageset to sphinx docs
Merge pull request #2907 from skirpichev/imageset-uneval

imageset() improvements
Workaround Dummy's printing in Subs constructor (fixes #7068)
Add test for issue 7068