Resolve "Update to OGSv6.4.1"

Closes #51

See merge request !33

Resolve "Accept a list of values in property/value"

See merge request !39

Add CI/CD Pipeline to check if benchmarks work with OGS version 6.4.1

See merge request !37

# Conflicts:
#   .gitlab-ci.yml

[ci] try custom gitlab-runner

[ci] test privileged docker envinf1

[ci] bm-test for v6.4.0

[ci] parallel ref and test benchmark runs

[ci] run benchmark test for version 6.4.1

[ci] final changes

[ci] check if package can be loaded in second stage

[ci] check for singularity

[ci] singularity & go installation

[ci] singularity setup in separate bash script

[ci] test custom container

[ci] 2 stage test custom container

[ci] remove installation of vtk and miniconda as already available in container

[doc] r2ogs6 uptdate protocol in doc/ folder

[doc] documentation for the updated classes

[doc] documentation for new keywords

[doc] documentation for prj_com_property

[base] update classes for ogs v6.4.1

[base] added class arguments for new keywords

[tests] skip benchmarks with *.xml project-files

Resolve "Add security routine for output file import"

Closes #47

See merge request !32

small test for directory without any *.pvd files

[docs] examples polished

Closes #46, #49, #48, and #50

See merge request !31

    paths to referenced files in the PRJ file have to be relative
    to the PRJ file

   before the complete config was replaced by the cfg file
   now, only specific options defined in the cfg file are updated

    building vignettes with devtools::built() failed as
    there is no ogs executable in the build path
    vignettes were knitted with knitr only

   commands updated
   relative paths used
   env variable used to enable executoin of ogs simulations

    unused examples removed
    examples that are described in vignettes removed

[tests] update benchmark PRJ files to ogs6.4.0 for sim-utils

Closes #45

See merge request !30