Started port of polyprism to Cython

Moved functions to a _polyprism_numpy module and made tests against the
main polyprism module (for now, with the same functions).'
Benchmark for polyprism
Started some code for polyprism in Cython.

Just copied the gz function over.
Strangely Cython polyprism is 2x slower than numpy

Had to replace 'f' dtypes in mesher for numpy.float.
Was slow because I forgot to declare a variable

Now the speed matches.
Converted all polyprism to Cython

The speed is equivalent now to the original (a bit faster actually).
Merge branch 'kernel-functions' into kernel-polyprism-cython

Conflicts:
	benchmarks/gravmag_polyprism.ipynb