This way it's easier to convert between strings/enum in different places.

This solves the problem of depending on initialization order of PointGroupFactory and SymmetryOperationFactory.

Added documentation and some more tests.

Changed PointGroup interface. Changed unit test for 6/mmm, since
(1 2 3) is a special hkl in that group. The test now uses (7 2 3).

This is required as Qt4 is not completely C++11 compliant.  I think that these will no longer be needed when we move to Qt5.
Should only be required for files up to and including API.
Refs #7284