Document the BEAST file formats

[karllark]

The BEAST file formats need documentation. This includes the output files with the fitting results (stats, pdf1d, and lnp) and the "internal" files (sed.grid, spec.grid, noisemodel, etc.). Very sparse docs in beast_grid_formats.rst. Related to #396 and #299.

Specifically:

• isochrone grid
• spectral grid
• sed grid
• observation model grid
• fittings results (stats, pdf1d, lnp)
• observed data file
• AST inputs file
• AST outputs file