Check that release site regions are instantiated.

Previously, we didn't check that region expressions which
describe release sites are on instantiated objects. The
new code fixes this. This addresses issue #6 on the github
tracker.
Merge branch 'master' of https://github.com/mcellteam/mcell
Improved check for instantiated release regions.

My previous commit wasn't 100% correct. Hopefully
this one is - it does pass the test suite.