FluidProperties is a process independent container
for common fluid properties like density, viscosity
and specific heat capacity.

This allows for correct documentation of the process_variables.

Automatically with sed
sed -i 's/ogs_file_param_attr{process/ogs_file_param_attr{prj__processes__process/g'
and similar to the ogs_file_param_special and the ogs_file_param
doxygen commands.