Resolve "New element "dvalue" in "media/medium/properties/property""

Closes #65

See merge request !46

Resolve "Fix manual pipeline"

Closes #64

See merge request !44

running ogs as singularity container in the gitlab ci
requires to run the ci-container (docker) with elevated
priviliges. this is accomplished via a manual login.

Resolve "Throw Warning when finding an empty XML tag"

Closes #55

See merge request !43

standardize files references during prj import/export (new MR)

Closes #57, #58, #59, and #62

See merge request !41

    GML, DAT, PY, PRJ includes, VTU files

    References for gml, dat, mesh vtu files that are exported to the prj
    file before simulation are standardized to use absolute path. There
    is an option to copy these files into sim_path; file references are
    then made relative.

    When an ogs6_obj is created from a prj file, the prj file may contain
    references to external files. This makes shure file references (gml,
    meshes, dat, py) have absolute paths. Then it is more easy to handle
    file references when sim_path and prj_path differ.