Add when validator for year parameter

[hdoupe]

• Adds the when validator that @jdebacker set up in update start year for CS Cost-of-Capital-Calculator#344.
• Extends the dump method for meta parameters to use the choice validator so that C/S will still show the dropdown for the year meta parameter even though it is actually using the when validator. This is necessary until C/S adds support for the when validator in the GUI.

Resolves #176

[hdoupe]

Sorry, accidentally formatted the entire test_functions.py file.

[hdoupe]

Well, validation works:

>           raise self.validation_error
E           paramtools.exceptions.ValidationError: {
E               "errors": {
E                   "year": [
E                       "When data_source is equal to CPS, year value is invalid: year 2013 < min 2014 "
E                   ]
E               }
E           }

The grid logic for testing get_inputs with all combinations of meta parameters doesn't handle the when validator correctly. I'm inclined to skip this test or input configuration somehow so this doesn't hold things up, but I'll think about this some more.