Detailed documentation of scattering math and rotations

HoloPy uses some formalisms for scattering that differ significantly from those appearing in standard references on scattering, e.g. Bohren & Huffman. In particular, HoloPy calculates amplitude scattering matrices that depend on the radial coordinate kr, instead of assuming an asymptotic spherical wave dependence. This is relatively straightforward for the Lorenz-Mie solution, but is not so clear for Multisphere. Detailed documentation for advanced users should be written to explain this stuff, as well as the rotation matrix formalism.