small test for directory without any *.pvd files

    set ogs executables explicitly instead of ogs bin folder path
    add option to use ogs singularity containers

Minor corrections added, documentation updated. Slowly sorting out how the .prj file parameters need to be exported. This will still take some time.

functionality to add the .prj parameters and thinking and what the
user will need to define and what can be added automatically. Added
(dirty prototypical) function to call the generateStructuredMesh.exe given the /bin path.