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