fix a logm problem, add fractional matrix powers, add references and tests

This PR adds a couple of Pade approximations of matrix functions, using inverse scaling and squaring.  I implemented these from the reference publications listed in the docstrings.  One of the example matrices in one of the publications causes problems for the current logm, and the new logm can handle it better; this example has been included as a test.

The way that the imports are organized might need to be changed, so suggestions about how to do this better (and any other suggestions) would be welcome.  If this PR is merged, `logm_new` could possibly replace `logm`.  Also, `fractional_matrix_power` might not be the best name.