enable "fabsim" command to be run from anywhere

[djgroen]

This has been a gripe from alpha users since M9, e.g. one listed the following as an annoyance:
"Maybe (at least for FabSim) the need to be in the right repository to run commands and not being able to do it from any place on the computer's hard drive."

With the development of bin/fabsim we can finally sort this problem, as I now configured it such that it will automatically load the fabfile from the scripts' parent directory, even when it is invoked from elsewhere.

However, since the fabsim command emerged so close to the M12 deadline, and it is so central to the toolkit functionality overall once we release it, we will urgently need to have some basic integrations tests around this, e.g. to see:

• if the fabsim -l command works as intended under CI.
• if the path to fabfile in the script bin/fabsim is correct under CI.

@orbitfold or @arabnejad are you perhaps able to make these tests somewhere later today?

[djgroen]

24 hours later, everybody seems to be happily able to use the new command, so I'll consider this a success for now. Case closed.