[Element] Restricted Diffusion Element #269

Please submit a reference and the equation for a proposed element! Attaching an example data set is also usually helpful :)

Element: Restricted Diffusion Element (aka finite-length diffusion element)

Reference: Vicente, N.; Haro, M.; Garcia-Belmonte, G. New Approaches to the Lithiation Kinetics in Reaction-Limited Battery Electrodes through Electrochemical Impedance Spectroscopy. Chem. Commun. 2018, 54 (9), 1025–1040. https://doi.org/10.1039/C7CC08373D.

Picture of Equation (Equation 5 in the reference):

This element is one of the options in EC-lab (there it's called the modified restricted diffusion element). It's the more general case of the open (finite-space) Warburg element in impedance.py, with the parameter gamma added to account for non-ideality.