Dimensional constraints (backend version 0.21)

[MilesCranmer]

TODO:

• Allow unicode printing
• Integrate units.
  • We probably want to pass these as strings, otherwise we'd need to make a converter from some Python units package. Also astropy doesn't allow you to have different units in the same numpy array, so seems perhaps not optimal for this.
  • X_units and y_units parameters to .fit()?
• Add print_precision parameter.
• Add dimensional_constraints_penalty
• Consider taking advantage of v_dim_out parameter.
• Reach 100% diff coverage (see todo in tests file)
• Test selection mask + units
• Test pickling + units
• Test re-fit + units
• Add parameters to param groupings
• Set y_variable_names

Fixes #283, #388