add documentation for ThermoRichardsFlow

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/c_THERMO_RICHARDS_FLOW.md

+\copydoc ProcessLib::ThermoRichardsFlow::ThermoRichardsFlowProcess

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/process_variables/i_process_variables.md

+The process variables for temperature, pressure.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/process_variables/t_pressure.md

+Process variable name for pore pressure.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/process_variables/t_temperature.md

+Process variable name for temperature.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/t_coupling_scheme.md

+Coupling scheme. So far, only the full monolithic scheme is available.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/t_mass_lumping.md

+A tag for enabling diagonal lumping of the mass matrix of the Richards equation.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/t_simplified_elasticity.md

+Considertion of mechanics in the mass balance equation under specific stress conditions
+to approximate the \f$ \nabla \cdot \dot {\mathbf u}\f$-term. See \cite Buchwald2021 for details.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_FLOW/t_specific_body_force.md

+\copydoc ProcessLib::RichardsMechanics::RichardsMechanicsProcessData::specific_body_force

Documentation/bibliography/other.bib

@@ -10,7 +10,17 @@
   Year={1964},
   Publisher={colorado State University}
 }
-
+@Article{Buchwald2021,
+title = {Improved predictions of thermal fluid pressurization in hydro-thermal models based on consistent incorporation of thermo-mechanical effects in anisotropic porous media},
+journal = {International Journal of Heat and Mass Transfer},
+volume = {172},
+pages = {121127},
+year = {2021},
+issn = {0017-9310},
+doi = {https://doi.org/10.1016/j.ijheatmasstransfer.2021.121127},
+url = {https://www.sciencedirect.com/science/article/pii/S0017931021002301},
+author = {J. Buchwald and S. Kaiser and O. Kolditz and T. Nagel}
+}
 @Article{Ehlers1995,
   Title={A single-surface yield function for geomaterials},
   Author={Ehlers, W},