Shared memory parallelism with OpenMP in Cython

Testing the OpenMP support in Cython.

# Initial tests

Using `prange` in `gravmag.prism.tf` resulted in ~3.5x speedup with 8 threads. The `prange` loop is in the data loop (inside the model loop). See [the full benchmark](http://nbviewer.ipython.org/github/leouieda/fatiando/blob/openmp/benchmarks/gravmag_prism.ipynb).

Need to test how this works with `multiprocessing`.

# Checklist

- [ ] Cookbook recipes
- [ ] Docstrings
- [ ] Doctests and unitests
- [ ] All tests pass
- [ ] Can be merged
- [ ] Changelog entry
- [ ] Benchmark against old implementation