Mass coefficient if $DENSITY model 2 used

[lurpi]

Hi all, I think there is a missing condition in the if here: https://github.com/ufz/ogs5/blob/8b235b2dc7ee43b3df6ea9e29ebe95b7a0e97b70/FEM/fem_ele_std.cpp#L1581

If fluid compressibility is taken into account via $DENSITY (without $COMPRESSIBILITY) then the resulting mass coefficient lacks the "divided density" term. Best,

Luca