Changelog entry for kernel functions and Cython

New separation of gravmag forward modeling into kernels and Cython
implementations of sphere and polyprism
Better docstrings for gravmag.prism
Merge branch 'kernel-functions' into kernel-openmp
Benchmark prism and sphere against numpy code
OpenMP for gravmag.polyprism

Benchmarks are very good! Sometimes more than 2x speedup over numpy
version.
Moved the vertices loop out of kernels to avoid segfaults.
Had to change the way PolygonalPrism makes .x and .y attributes so that
they are C contiguous arrays.