Stockpyl

Added
- Methods to convert objects to and from dicts

Changed
- Removed dependency on pandas

Fixed
- Bug that prevented named instances from being loaded correctly using ``load_instance()``

Changed
- Removed cost summary output and ``print_cost_summary`` option from ``sim_io.write_results()``

Fixed
- Small issue in README file

Added
- Various new unit tests

Fixed
- Bug in ``supply_chain_network.echelon_to_local_base_stock_levels()``