Rename `MatrixGroup` / `MatrixGroupElem` to `MatGroup` / `MatGroupElem`

[fingolfin]

As @JohnAbbott recently point out, it is somewhat confusing that we have MatElem and MatRingElem on the one hand, but MatrixGroupElem on the other hand.

Even more so given that there is also MatrixElem which is an alias for Union{MatElem,MatRingElem}.

Now in the grand scheme of things, this is not very important. But I guess we could rename the group types? Keeping the old names as aliases of course for backwards compatibility.

[thofma]

(During the "Great Renaming" we also changed the matrix algebras to use MatAlgebra.)

[ThomasBreuer]

Good idea.
Concerning a global replacement of the string MatrixGroup by MatGroup, this would also affect a module name MatrixGroups in experimental, but otherwise there is no problem.