Add Ability to Update Beam Position in `GNuMIFlux.xml`

[alexbooth92]

In the old, now deprecated, production workflow we had the ability to make "off-axis" samples. This was never ported to the modern workflow for two reasons:

• the method is an approximation, we simply move the position of the beam with respect to a fixed ND hall, we don't move NDLAr and TMS within the hall.
• it hasn't been a priority for the wider group.

There is now interest in off-axis samples again and, despite the limitations in the method, samples produced in this way are very useful.

Here is the ancient code block. This logic would need to be introduced somewhere around here in a way that's consistent with our current method for configuring parameters at run time via env var.