1.7.0

Remove Persistor's query_by_keys() and df_from_query_by_keys(). Use query() and df_from_query() with keyword argument keys and parameters.

Add caching to Persistor's df_from_query() with keyword argument cache as the filesystem location to store compressed picked dictionaries of sql -> DataFrame. Caching compares the literal rendered sql only: it is not aware of volatile functions called on the server (like current time) or side effects (like temp tables).

Add rendering of multi-column key values to Persistor's query() and df_from_query(). Users pass DataFrames instead of single column sequences. See test/test_union_all.py