Improves flexibility across instruments instead of writing
more code.
Refs #26639

Found via `codespell -q 3 -I ../mantid-word-whitelist.txt` where whitelist consists of:  
```
alloced
allws
ang
ans
arithmetics
dum
dur
iff
isnt
ith
lod
mut
nd
originaly
sav
splitted
te
tim
uint
```

Facility has to be set to 'ILL' in testsetup.

Re #20453

Added a real IN4 dataset to doc test data dirs, and updated the
DirectILL* algorithms' docs to use that instead of the 'fake' IN4
workspaces. Also, upgraded the DirectILL concepts page to use the real
data.

Re #20453

Add documentation for DirectILLTubeBackground and update DirectILL technique docs accordingly.

Re #22224

This reverts commit 67938cfc.
I realised that this would be a good exercise for the next person
to take over isis_powder, so have left the commit in for reference

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