Adding colormap functionality

[odedstein]

Adding a function "colormap" that works like MATLAB's colormap, and then also "apply_colormap" to apply the colormap to a scalar function.
I was not sure what exact interpolation behavior to choose for "apply_colormap", but MATLAB does piecewise constant, so I went with piecewise constant.

[sgsellan]

I made a few minor changes. Maybe most important for standardization is that I changed the docstrings to better fit (my interpretation of) the numpy standard. I think I'd rather be strict about the standard unless we have a good reason to deviate from it. Feel free to revert if you have any strong feelings about this.

I also removed the dependency on matplotlib from our setup file and installation builds, and added some checks to the unit tests.

If you're happy, I'll merge after checks