Implement Drude Model

[tylerflex]

Want to allow user to specify (lossy) Drude model.

All Medium objects are defined here.

We want to implement a new DispersiveMedium for Drude model.

Note that the frequency-dependence of all mediums is computed through the Medium.eps_model(frequency) function, which returns the complex relative permittivity at frequency in Hz.

For PoleResidue medium, this model is just a function of the poles and eps_inf.
For other dispersive mediums, it will be a different function of the model's attributes, for example for Debye, the eps_model is a function of self.eps_inf and self.coeffs.

Note that all Mediums are handled by the solver as PoleResidue model. Therefore each DispersiveMedium has a pole_residue @property that returns a PoleResidue object implementing the same eps_model.

What to do:

• Create a new branch off from develop to implement the code changes (git checkout develop, git checkout -b drude)
• Add a new DispersiveModel called Drude following the template in Sellmeier, Lorentz, and Debye. See here for example:
• Define the additional input arguments needed to define the Drude model, their types, default values, and constraints. See this for discussion on simply constraining types of the model.
• Write an eps_model method for Drude using the model's attributes.
• Find the conversion from a Drude model to the eps_inf and poles of PoleResidue model and implement pole_residue() method.
• Add Drude to both dispersive material tests, which will test both creation and that self.eps_model matches self.pole_residue.eps_model (for consistency).
• Add Drude to the MediumType to specify that it can be used as a Medium in the rest of the project.
• Edit docstring and add Drude to the API.
• commit and push your changes to drude branch, open pull request and assign Tyler for review.