feat (source): make version separator configurable

[simbuerg]

This makes the revision separator configurable by the settings file.

Option:

BB_VERSIONS_SEPARATOR

Default: ','

[LuAbelt]

While using this PR, I noticed that the run command line still expects a , as a separator when I want to run a project at a specific version.

From first glance this seems to be due to the __add_filters__ implementation. Not sure if that is the desired/intended behavior; It just seemed a bit odd to me.

[LuAbelt]

Some further observations/effects of that:

When generating a slurm script via the benchbuild cli, it will generate the project list in the script the the separator defined in the settings file.
Due to that, the subsequent benchbuild call in the script will get the "wrong" version of the project and not execute any experiment