Can't unpickle matrix groups with no generators

[ChrisJefferson]

In GAP, we can make groups with no generators, so calling Group(GeneratorsOfGroup(g)) doesn't work. The fix is to pass the identity of g as the second argument of 'Group'.

I've fixed this for permutation groups, but not for matrix groups, as nothing simple seems able to fix It:

• The way the pickling is set up, I need the group before I know the dimension and field.

• Passing in a "fake" list of generators which contains the identity is also tricky.

We could just make a fresh matrix group where "GeneratorsOfGroup" won't return nothing, or change how we pickle them so we always pass the identity in (changing the name of course).