allow data loading from mutliple archives

Presently data is organized in one folder (archive) with folders for monthly archives each containing an index and daily parquet files.

Data is presently loaded by providing a path to the monthly archive, reading the index file and using it to loop over daily parquet files for concat'enating data. The output is a pandas.DataFrame containing a requested data for a time interval.

Loaded data is presently bound to come from the same month.

A better solution requires an indexer class that can be instantiated from the path of the top level archive. it would read all index files found and can, given an time intervall, provide a list of pathes to daily parquet files on which to loop.