Addressed Issues #6 and #8.
Added _grow_matrix() that operates as expected.
Collapsed large sections of _traversal() to increase modularity.

Note that the current implementation tests tolerance on the final
isotope AFTER destruction is applied.
Added test for matrix grower method (#10).

Fixed test_expm to use correct numpy array equivalency test.
Unfortunately, this also removed the high level Taylor series convergence
test. Since the taylor method is not currently in use, this is an acceptable
loss.
Merge branch 'transmute' into staging