Add support to read and write Amber restart files

This supports reading and writing both NetCDF and ASCII Amber restart files, and includes `load_restrt` and `load_ncrestrt` functions to load them as `Trajectory` objects.
Avoid == Above?  I thought a `ValueError` was appropriate here since if it reaches this point, the file is clearly a NetCDF file.  I thought a `TypeError` was appropriate if it wasn't a NetCDF file.  It's a `ValueError` if it _is_ a NetCDF file that doesn't have the necessary information.  At least that was my reasoning for doing what I did.

I can switch it to a `TypeError` if you want.