Add file based sub blocs to `dat` bloc

Currently the phreeqc.dat files is read in and a dat bloc is created as ogs5_obj$input$dat. However, there are various *.dat files defined in other benchmarks, e.g. C/calcite_gems/calcite. The phreeqc.dat is a phreeqc specific file.

Two solutions:

a) The corresponding dat bloc should be created at ogs5_obj$inpu$pqc$dat. It might be best to have a sub bloc for the *.pqcfile ogs5_obj$inpu$pqc$pqc and one for *.dat file ogs5_obj$inpu$pqc$dat This seems logic from the point of where the data in the file belongs.

This should then be similar for other blocs that use * .dat files.

b) Create a ogs5_dat class for *.dat blocs. Create the corresponding phreeqc.dat bloc at ogs5_obj$input$dat$phreeqc, using the base filename (here: phreeqc) as bloc name. Do the same for other *.dat files.