[HM] benchmark staggered scheme, use unit mD precisely and correct nondim time

A further review revealed a discrepancy between the time scale of the results with the reference [Kim]. Indeed, the nondimensional time refers to the pore space filled and not the full domain. Thus the characteristic time changes from 1500 days to 450 days. In addition the exact value for the permeability unit Darcy is now used instead of the approximation of 1mD=1e-15.

Tests and documentation are updated.