'expand to full parameter array when some parameters are fixed
Parameters
params : array
reduced parameter array
Returns
paramsfull : array
expanded parameter array where fixed parameters are included
Notes
Calling this requires that self.fixed_params and self.fixed_paramsmask
are defined.
*developer notes:*
This can be used in the log-likelihood to ...
this could also be replaced by a more general parameter
transformation.'

def expandparams(self, params):

    paramsfull = self.fixed_params.copy()
    paramsfull[self.fixed_paramsmask] = params
    return paramsfull
