Changes

Pre-Registration (versions <0.1.0)

Breaking:
- Fixed PNP discretization: for consistency with equilibrium we need to use $\bar c \nabla \frac{c_i}{\bar c} \dots$ instead of $\nabla c \dots$.
- ivsweep and dlcapsweep now have their own return types. These can be amended by more information without breaking codes
- ivsweep and dlcapsweep have now a store_solutions keyword argument (false by default) which indicates if all solutions should be stored.
Introduced Poisson-Boltzmann solver
Surface species example (thanks @smaasz!)
Checked consistency with equilibrium for all variants (equilibrium, pb, pnp)