Gas Pressure Dependent Permeability : MPa changes in gas perm

Hier ist fixing the problem in gas pressure dependent permeability model described in issue number !3439 (merged).

The code requires some modification, since the formula used in the Reference Paper Xu et al. (2013) had a MPa unit for the pressure (i.e., /1.0e6). double const factor = (gas_pressure <= pressure_threshold) ? (1.0 + a1 gas_pressure/1.0e6) : (a2_ (gas_pressure - pressure_threshold_)/1.0e6 + 1.0 + a1_ * pressure_threshold_/1.0e6); /ogs/MaterialLib/MPL/Properties/GasPressureDependentPermeability.cpp @kuateric