Rearrange sans function to allow changing the tolerance Seek centre

Inside ISISCommandInterface.py, change the FindBeamCentre in order to allow
providing a new value for tolerance. It now, uses the previous values
as the default input.

re #7824
Export the tolerance option to GUI SANS interface

re #7824
Alert user for invalid input for Tolerance

re #7824
Merge remote-tracking branch 'origin/feature/7824_seek_centre'