This change applies to diameter models within the multiphaseEuler
module, heat transfer fvModels, and the LopesdaCosta porosity and
turbulence models.
User input changes have been made backwards-compatible, so existing
AoV/a/Sigma/... entries and fields should continue to work.
Given that the number of solid solver modules is currently 1 and unlikely to
exceed 3 it is not very useful to maintain solid and fluid sub-directories and
easier to see the correspondence between the solver modules and tutorial cases
without.
