New CLI option for saving summary file

The main change is that there is now a CLI option for saving a summary file (of user inputs and calculated effective mass values). This is important so that there is a record of the calculation done (to enable reproducibility).

The other change is that dependencies are handled more efficiently. The packages needed for testing or docs are only available when specified (using pip install effmass[tests] and pip install effmass[docs]) and are not required for standard install.