Introduce SolidSystem and SolidComponents
The SolidSystem calculates quantities like solidHeatCapacity, solidDensity and solidHeatConductivity based on the compositions
of individual solid components such as Granite or Sand.
The concept uses the components' volume fractions which are either solution dependent or fixed via the spatialParameters.
For simple cases, where there is only one non-reactive solid component, the base spatialParams implement a volumeFractions() method that calls the spatialParams implementation's porosity() method and returns 1-porosity.
For other cases, this method throws an error and therefore forces the user to implement an own volumeFaction() in the spatialParams.
The solidSystem has an interface function porosity() depending on the solid composition, i.e. a SolidState.
This may be realized using some template meta programing magic.
Related to #452 (closed) #433 (closed) #450 (closed), fixes #409 (closed).