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.
Maybe.  What do you propose?  Warnings?  `ValueError`s?  There are a very small number of programs that currently write these files.  And the file is not written correctly unless these attributes are present.