Introduced density for mass-based formulation in StaggeredHM
- Multiplication with density was missing in mass balance of HM_staggered_scheme, this has been corrected.
- Existing tests passed, except one, but this test (HydroMechanics_hm1_2Dsquare) is not affected by the changes made.
- It is a Bugfix, no feature was added, so there is no documentation.