Document available software on scalc*, scalcg* and ursus7
Currently, the software installed on these machines differs. Examples,
- MATLAB is available on ursus7, but not on scalc* and scalcg* (see here for a Singularity solution)
- Singularity available on scalc* and scalcg*, but not on ursus7
As this is a source of confusion, documentation on what software is available should be added.
/cc @sabine-haase