add more complex model constraints

[sdtaylor]

The phenograss model especially, need various constraints on the parameters. This is done to some to extent in the fitting by specificity parameter ranges, but sometimes more is needed. ie see here 7787263

Potentially add a model method for this

def check_constraints(self):
    Phmax > Phmin
    ...


# inside the mode fitting
if self.constraits_met:
    raise warning
    V[:] = 1e10
    return V