Ambiguous default value initialized for params.forcing.key_forced

Created originally on Bitbucket by avmo (Ashwin Vishnu)

I had assumed for a long time that the default key_forced was initialized based on the value set for params.forcing.type. Apparently that is not the case, since it depends on which class is initialized last.

This can lead to unpredictable behaviour when a new Forcing class is included.

Relevant code.

#!python

class ForcingSW1L(ForcingBasePseudoSpectral):
    @staticmethod
    def _complete_info_solver(info_solver):
        """Complete the ParamContainer info_solver.

        This is a static method!
        """
        classes = [Proportional, TimeCorrelatedRandomPseudoSpectral, Waves]
        ForcingBasePseudoSpectral._complete_info_solver(info_solver, classes)

I propose another hidden parameter which is like params.forcing.waves._key_forced from which params.forcing.key_forced is intialized during runtime.

It should be something like:

#!python

if params.forcing.key_forced is None:
     tag = params.forcing.type
     params.forcing.key_forced = params.forcing[tag]._key_forced