Implementation of current diffusion

* Timescale of current diffusion makes physical sense. But no detailed benchmark against RAPTOR/JETTO/ASTRA yet done
* Includes resistivity_mult option to accelerate (when >1)/slow-down (when<1) current diffusion. Convenient for testing and bringing current diffusion timescale more in line with energy confinement timescale
* Bootstrap current not yet included
* External current settings still constant throughout simulation

Ion and electron heat transport. Circular geometry.
Fipy library for implicit + explicit timestepping
Numpy explicit for testing and comparisons