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.

PointGroup objects can now be constructed through the factory using the Hermann-Mauguin symbol.

For most symbols this was just removing spaces, but for monoclinic, I decided to have the "usual" case with unique axis b as 2/m and the c-axis should be unique, this has to be made explicit.

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).