Make more dependencies optional

Is it worth switching to on demand loading of things like scattering theory fortran modules? This would allow people to import holopy and, for example, use the reconstruction code, without needing to figure out fortran compilers.

see holopy.vis for an example of how we are doing this with matplotlib and mayavi for visualizing clusters and holograms. Also scatterpy.theory.dda does this with a-dda.