traci_2_1_spreadsheet

This provider needs some work: - the open_workbook call should not happen in the constructor, but in _load_all - then the provider should be marked 'static' - that would enable the init to be instantaneous, and the load to occur through normal channels in LcResource - that would also allow the loaded result to be cached in JSON = model: Ecoinvent LCIA

This suggests that maybe there should be a versioned LCIA base class-- probably in the data_sources hierarchy and not the providers hierarchy though... On that topic, it is simply foolhardy to try to build a version-agnostic provider class in the ABSENCE of multiple versions (as is the case with TRACI)