Add interface to model atmospheres

[wtbarnes]

#408 added parsers for all of the new v11 files, including the model atmospheres that can be used in the charge transfer calculations for ionization and recombination. However, there is currently no interface to these model atmospheres (other than reading them directly from the HDF5 file).

Unlike almost all other datasets, these data are not associated with any element or ion so it does not make sense for them be attached as datasets to Ion or Element. Maybe these atmospheres should just be accessible via a simple function that returns a table? This should be relatively easy to do as the DataIndexerHDF that acts as the interface to all datasets in the HDF5 database has an as_table method. Thus, it should just be a matter of creating a wrapper function around an indexer that pulls a specified model atmosphere.