IsMinimalNonmonomial only implemented for solvable groups

[ssiccha]

Reported by Bill Allombert via the GAP forum mailing list.

We have two options, taken from an Email by @ThomasBreuer to the GAP forum:

• Implement a mehtod for nonsolvable groups. Just providing some simpleminded code just in order to get an answer instead of a "no method found" error message is not a good idea. If someone is really interested in the question for nonsolvable groups then probably the first step is to ask for minimal nonsolvable simple groups, and then one looks at the monomiality condition for the subgroups of these groups.

• Change the documentation such that it says that GAP can currently answer this question only for solvable groups and give a more informative error message if the argument is a nonsolvable group. An example for such a solution is the function Omega, which states: At the moment methods exist only for abelian G and n=1.

Option number 2 should be relatively easy to do, which is why I'm marking this
as good first issue.