vc: added param checks to new routines
vc: added test for _ensureOneHead
vc: added test to show _ensureLocalPerspective can handle multiple heads

added test to show _ensureOneHead does not fix multiple heads
vc: _branchHeads has now possibility to omit conflicted versions
vc: added routine _markConflict and implemented _ensureOneHead to work with it + test
vc: clarified some function descriptions
vc: added _ensureLocalPerspective2, _markConflicts, changed _ensureAllInDAG

Duplicated _ensureLocalPerspective to return the versions that already exist
so we can continue with the process. Removed extra _ensureOneHead in
_ensureAllInDAG. Also now marking conflicted heads.