Added a basic IP algorithm.  It's 10x faster than James' IP_with_timing.  Needs a bit more work such as a return_results method and docstrings.
Updated docstring in tortuosity
find_neighbor_pores and _throats were sending back ndarrays most of the time, but empty lists [ ] when no neighbors were found.  This was breaking things so I forced arrays.
Fixed bug #254: There were several places using if pores != [ ] that were changed to pores is not [ ]
Fixed bug #242: During the _labels method, arrays were being made based on the size of pores, which worked great when pores was indices, but when pores were a mask, the size was always Np.  This now converts to indices first.
Tiny change to _get_indices so that always returns an int32 array.  For some reason, sp.where returns an int64 array.