FromHoudiniGroupConverter ignores internal groups.

This fixes a bug where the group converter would take preference just because of the selection state. I've verified the fix, but unfortunately I can't write a test case, because there doesn't appear to be any way to create internal groups from python...
correctly ignoring internal groups
Merge pull request #77 from andrewkaufman/convertSelectionBug

FromHoudiniGroupConverter ignores internal groups