v0.16.0

Major changes:

• Update species.mass field to new value for OPTIMADE API v1.0.1-develop (#213).
  The value is now a list of floats that should match up with the elemental mass in a.m.u. from the species.chemical_symbols list. In order to be compliant, species.mass is now either a list with one or two (in the case of a vacancy) value(s) or None.
• Update to optimade v0.13.0 (#213).

Minor and development changes:

• Use while instead of if for removing end-of-line URL slashes (/) (#213).
• Properly report exceptions and errors for CLI commands calc and init (#213).
• Update README with instructions for testing the MongoDB backend (#213).
• Use complete path imports from aiida (AiiDA-Core) instead of wildcard importing from aiida.orm (#213).
• Properly test and cover code for initializing structures with partial occupancy (#213).
• Update various dependencies (#214).