Merge branch 'BHE' into 'master'

ProcessLib/HeatTransportBHE/BHE/BHE_1U.cpp

ProcessLib/HeatTransportBHE/BHE/BHE_2U.cpp

Tests/Data/Parabolic/T/3D_2U_BHE/3D_2U_BHE.prj

Tests/Data/Parabolic/T/3D_2U_BHE/3D_2U_BHE_powerBC.prj

...ta/Parabolic/T/3D_2U_BHE/3D_2U_BHE_ts_10_t_600.000000.vtu

..._BHE_GW_advection/BHE_GW_advection_ts_10_t_500.000000.vtu

...c/T/3D_Beier_sandbox/beier_sandbox_ts_10_t_600.000000.vtu

..._sandbox/fixed_power_constant_flow_ts_10_t_600.000000.vtu

web/content/docs/benchmarks/heat-transport-bhe/3D_2U_BHE.md

...t-bhe/3D_2U_BHE_figures/In_out_temperature_comparison.png

...e/3D_2U_BHE_figures/vertical_temperature_distribution.png

...BHE_figures/vertical_temperature_distribution_powerBC.png