Remove some duplication in asset retrieval code

[mcgibbon]

Some of the code introduced by #27 could be reduced by writing generic functions that can be re-used for the different types of run data (e.g. orographic, input, forcing...). This would improve maintainability and make it easier to generalize the code down the line for different models.

[oliverwm1]

Ditto for the get_directory and get_table_filename functions in _datastore.py and _tables.py.