MPL: distiguish between volumetric and linear expansion coefficients
While in some cases the (fluid) thermal expansivity is derived from the density, most processes still take the expansion coefficient as an extra property (or both). However, when given as extra property, ogs/MPL uses implicitly the convention that the fluid property is a volumentric expansion coefficient, while the solid property is given as a linear expansion coefficient. From a theoretical point of this behavior is not obvious as both coefficients do also mix in the mass balance equations. Therefore, I would suggest the following name changes: fluid: thermal_expansivity->volumetric_thermal_expansivity solid: thermal_expansivity->linear_thermal_expansivity What do you think?