Muons: access scripts loaded via g.load_script from the reduced namespace

As a muon instrument scientist, I would like to type `r.runscript()` rather than `runscript()` to execute a script which has been loaded from the script generator via `g.load_script()`. This is because I want my users to always have a consistent set of commands - i.e. all commands begin with `r.` and end with brackets.

### Notes:
- Currently `g.load_script` loads scripts into the global namespace.
- We do need to consider the case when no script has been loaded, and give a sensible error message.
- The reduced set of commands (`r.`) was implemented in https://github.com/ISISComputingGroup/IBEX/issues/5066