add submodels for effective parameters

[brosaplanella]

Summary
Currently, the Bruggeman correlation is hard-coded into the model. We should make it more general by introducing a parameter that accounts for the effective properties of the material similar to the curly B in Richardson et al. (2012). As discussed in the meeting (07/10/19) we could introduce a submodel for it (called porosity, for example) which calculates this parameter from the other ones.

Motivation
Bruggeman correlation is widely used in battery modelling, but it is only one empirical approach to determining the effective properties of a porous materials. There are other approaches such as asymptotic homogenisation or softwares like TauFactor so a more flexible definition of the porosity would allow to use these other techniques.