Implemented an algorithm for finding the center of a permutation
group using .centralizer() in a straightforward way.
Moved the DirectProduct() function to a new file,
The commutator subgroup, also known as the derived subgroup,
is the group generated by all the commutators of the group.
The reason for the namechange is that the name commutator
will be used for a more general procedure.
Started writing tests for the new functions (currently, there are
tests for .center() and .centralizer().
Wrote some new utility functions to help in the test process.
Wrote a method to test whether a permutation group is trivial.
Added some more tests for the new functions and created a new file,
sympy/combinatorics/testutil.py that will accomodate utility functions
used specifically in testing functions from the combinatorics module