Create a separate Python/C helper library to remove code duplication

Fix remaining failing system tests

InstrumentMaskTab Exception Fix

Fix for ISISPowder_Gem_Focus_Test

Sans table change feature branch

Display all valid file extensions first

docs: typo fixes

MSVC was not copying sipqti.cpp.in when it was updated. This
simplifies the rule set to avoid needing to copy it.

Use the new PythonInterfaceCore library to avoid code
duplication.

This will contain code commonly used by any part of the code
base that interacts with the Python C api.

Bugfix for DeltaE>Ei in DgsPlanner

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
```

MR: add option to overwrite DANGLE0 and DIRPIX

Move back to correct algorithm

Add support for masking workspaces to MaskDetectorsIf

Expose the TSVSerialiser to python

Fix local links in dev-docs