ENH: surfaceFilmModels::waxSolventEvaporation, waxSolventViscosity: new wax/solvent film models

to support the evaporation of the solvent from the wax film and the changes in
viscosity caused by the reduction in solvent content.

BUG: filmViscosityModel::thixotropicViscosity: Corrected sign of impingement rate

to compensate for rhoSp having the wrong sign

BUG: surfaceFilmModels::waxSolventEvaporation: Corrected handling of impingement

ENH: surfaceFilmModels::waxSolventViscosity: Changed mixing to mole-fraction based

ENH: surfaceFilmModels::thermoSingleLayer: Added call to solveContinuity before updateSubmodels

to allow sub-models to solve transport equations for conserved properties