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.
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, sympy/combinatorics/group_constructs.py