Add Improved Transfer Functions from Rodriguez et al.

[BradyPlanden]

Implement the transfer functions derived in https://doi.org/10.1016/j.est.2018.06.015. These functions remove the current assumption of electrolyte potential is non-dependant on electrolyte concentration.

• Refactor transfer function selection in dra.jl
• Implement transfer functions
• Align cell parameterisation for new functions