Re-factor dataset (table) getters

[jonkeane]

We have dataset.get_table_dataset() which then calls dataset specification getters (get_raw_tpc_dataset_spec and get_csv_dataset_sped), but this probably isn't quite right. Would it be possible to simplify these so that they all get the dataset directly?

These also are a bit funny because in principle they should be methods on the Table class (but they require information about the dataset those tables belong to in order to work, so maybe we need to do some duplications of attributes to make that not be necessary?