[PL/TH2M] Switch to constant solid volume fraction

ProcessLib/TH2M/TH2MFEM-impl.h

ProcessLib/TH2M/Tests.cmake

...mpression/HM_confined_compression_gas_ts_0_t_0.000000.vtu

...sion/HM_confined_compression_gas_ts_120_t_1000.000000.vtu

...mpression/HM_confined_compression_gas_ts_1_t_5.000000.vtu

...ession/HM_confined_compression_gas_ts_20_t_100.000000.vtu

...sion/HM_confined_compression_gas_ts_420_t_4000.000000.vtu

...ession/HM_confined_compression_liquid_ts_0_t_0.000000.vtu

...n/HM_confined_compression_liquid_ts_120_t_1000.000000.vtu

...ession/HM_confined_compression_liquid_ts_1_t_5.000000.vtu

...ion/HM_confined_compression_liquid_ts_20_t_100.000000.vtu

...n/HM_confined_compression_liquid_ts_420_t_4000.000000.vtu

Tests/Data/TH2M/HM/flow_fully_saturated.prj

Tests/Data/TH2M/HM/flow_fully_saturated_gas.prj

...ata/TH2M/M/M_2d_neumann/result_TH2M_M_ts_0_t_0.000000.vtu

...ata/TH2M/M/M_2d_neumann/result_TH2M_M_ts_2_t_2.000000.vtu

..._1d_dirichlet/result_TH2M_T_dirichlet_ts_0_t_0.000000.vtu

...d_dirichlet/result_TH2M_T_dirichlet_ts_10_t_10.000000.vtu

..._dirichlet/result_TH2M_T_dirichlet_ts_20_t_200.000000.vtu

...dirichlet/result_TH2M_T_dirichlet_ts_30_t_2000.000000.vtu