Beam centre lookup table should perform linear extrapolation

[rtuck99]

The beam centre lookup table DetDistToBeamXYConverter.txt should perform linear extrapolation instead of reporting fixed values outside the lookup range.

See the following conversation at https://diamondlightsource.slack.com/archives/C048RD0KFFF/p1734351279634499

Steps To Reproduce

Steps to reproduce the behavior:

1. Set the detector distance outside the LUT range
2. The beam centre will be reported incorrectly in the nexus file and in ISPyB

Acceptance Criteria

• Beam centre is extrapolated correctly

[rtuck99]

We should take the opportunity to refactor the LUT logic slightly so that it can be placed in lookup_tables.py along with linear_interpolation_lut

[ndg63276]

Related to #522