Make HasSolvableFactorGroup slightly more efficient

[hungaborhorvath]

Old method checked if any element of DerivedSeries is in N. Now it only checks for the last element.

[codecov-io]

Current coverage is 50.47% (diff: 100%)

Merging #1062 into master will decrease coverage by <.01%

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[hulpke]

Yes, the current method was always just a quick hack.

However, if we're going to improve it, would't it make sense not to take the full derived series, but to iterate:

• Initialize D=G. Initially test that D>N
• Test whether the commutators of generators of D are in N. If so report success
• Otherwise compute D'. If D'=D return failure.
• If D <> D' then set D=D' and go back to step 2.

This way fewer derived subgroups are calculated, and in the last step the closure test is not needed.

[hungaborhorvath]

@hulpke I agree, but I did not see a nice way of doing that. As far as I see the DerivedSeries code needs to be duplicated (that has a lot of other nice exiting conditions, not only D=D').

Or maybe should be put into a global function which should be called both here and in DerivedSeries. I am happy to make that patch if that is the appropriate way to do this.

Edit: Never mind, I think I just said something stupid here. I will make a better patch about this.