Investigate if the beam centre is being used correctly at sans isis

[mantid-builder]

sans_bugfix
This issue was originally TRAC 7064

Original Reporter: Gesner Passos

While I was looking in detail to the sans isis reduction code, i found strange the relationship between set_beam_finder, get_beam_find_centre,
the reduction steps: _beam_finder and _front_beam_finder, and the pre_process inside sans_reducer that is called at the reducer.

I'm afraid, we are not correctly centering the HAB/front bank detector when this parameter is different from the defined for the rear/main.

Besides, the code inside WavRangeReduction after "for the LOQ instrument, if the beam centers are different, we have to reload the data." is to be properly explained and tested as well.

This ticket must, at least, define a proper systemtest.py to double check this issues. If they are correct beter ;)

[mantid-builder]

@NickDraper (2013-07-26T14:01:43):
Moved to the Backlog after R2.6

---

@NickDraper (2014-02-14T11:04:54):
Bulk move to assigned at the introduction of the triage step

---

@NickDraper (2014-03-07T16:20:29):
Ownership handed over to Peter

---

@PeterParker (2015-04-21T10:31:25):
This is for Richard to investigate the code (he wrote the parts responsible for this). Talk in a SANS Group meeting centred around whether or not the HAB in LOQ is connected to the rear detector.

[Matthew-Andrew]

This is resolved in #22372