Implement a new Plasma metaclass in PlasmaPy

PlasmaPy is an open-source Python package and aims to provide a set of common functionality used in plasma physics. Currently it implements a Plasma class which does all the fundamental work of calculating plasma parameters like alfven speed, density, electric field, magnetic field, pressure, etc. The current Plasma class does a good job at the moment but it will be very messy to work with different kinds of plasmas in the future by relying solely on current implementation using classes. Since dealing with different kinds of plasmas is an important part of plasma physics, having a more compact way of working with these kinds of plasmas will be a milestone of substantial importance and priority for PlasmaPy.