Problems when trying to set the non linear model

[SamuelBrieden]

I noticed, that if I want to set the nonlinear model via the class NonLinearModel (instead of directly using the set_params() function) CAMB returns me an error. I had a quick look into model.py and I think the problem can easily be fixed by replacing
self.NonLinear = self.make_class_named(initial_power_model, NonLinearModel)
by
self.NonLinear = self.make_class_named(non_linear_model, NonLinearModel)

The way it is at the moment it seems to be a typo (See the full function below)

`    def set_classes(self, dark_energy_model=None, initial_power_model=None,
                    non_linear_model=None, recombination_model=None):
        """
        Change the classes used to implement parts of the model.
        :param dark_energy_model: 'fluid', 'ppf', or name of a DarkEnergyModel class
        :param initial_power_model: name of an InitialPower class
        :param non_linear_model: name of a NonLinearModel class
        :param recombination_model: name of recombination_model class
        """
        if dark_energy_model:
            self.DarkEnergy = self.make_class_named(dark_energy_model, DarkEnergyModel)
        if initial_power_model:
            self.InitPower = self.make_class_named(initial_power_model, InitialPower)
        if non_linear_model:
            self.NonLinear = self.make_class_named(initial_power_model, NonLinearModel)
        if recombination_model:
            self.Recomb = self.make_class_named(recombination_model, Recombination
```Model)`