Added benchmark for gravmag.sphere
Updated prism benchmark
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.
Benchmark for gravmag.polyprism

Ran on an i5 laptop
Merge branch 'master' into kernel-functions
polyprism.tf is slower with the new kernel funcs
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
Merge branch 'kernel-functions' into kernel-openmp

Conflicts:
	setup.py

Changed the setup for building extension modules. Getting a list of
module names plus extra compile args.