[feature] Customizable low energy electronic stopping mode

[drobnyjt]

Is your feature request related to a problem? Please describe.
Currently, a user has the choice of four electronic stopping models: Oen-Robinson, Lindhard-Scharff (with optional correction), Equipartition (with optional Lindhard-Scharff correction) and Interpolated.

However, a recent paper suggests that the Lindhard-Scharff form of electronic stopping, dE/dx = k*S0*E^(1/2), may be improved by replacing the constant 1/2 with a customizable one. I've seen this before in other fits to experimental data, but this paper has served as a recent reminder.

Proposed solution
Introduce new form of low-energy electronic stopping of the form A*E^b, where A [eV/A or other compatible units] and b are user-specified constants.