Move McStas configuration out of notebooks

[jl-wynen]

Currently, McStas is configured in the notebooks with

my_configurator = ms.Configurator()
my_configurator.set_mcrun_path("/usr/bin/")
my_configurator.set_mcstas_path("/usr/share/mcstas/3.4/")

This is needed to build the book in CI. But is is incorrect on most client's machines. Including the schools JupyterHub. So move it from the notebooks somewhere else. E.g., into the CI script.

[willend]

In the school JupyterHub McStas is detected by McStasScript by means of the $MCSTAS env var and which mcrun.
(see https://github.com/PaNOSC-ViNYL/McStasScript/blob/6bbe6faf8129065cde82d3dac688ab0c6d5154c0/mcstasscript/interface/instr.py#L2997-L2999)

In e.g. conda the env var is automatically set. A relevant export MCSTAS=value could e.g. be probed via mcrun --showcfg=tooldir and set in in the CI after installation (assuming that mcstas/mcrun is on the PATH)