automatically compute parameters based on geometry

[rtimms]

Change parameter files to automatically compute surface area per unit volume based on particle shape, e.g. a = 3*eps/R for spheres. The plan is to make things like standard_parameters_lithium_ion.py classes so they can take options. For now I'll just compute surface area per unit volume, but there may be other things we want to automate based on options (e.g. cell volume based on pouch, cylinder etc.). Any suggestions welcome.