Creation of SANSWideAngleCorrection python api 2

re #6962
Update documentation SANSWideAngleCorrection

re #6962
Some improvements to SANSWideAngleCorrection

Progress reporting, and moving toward directives of python new api.

re #6962
Add unit test for SANSWideAngleCorrection

re #6962
Make the unit test independent of loading file data

it is required from the 'good practices' inside Mantid Team.

re #6962
Move documentation note to the usage section

The scientist suggested to put there. Now, that I'm aware of
WIKI_USAGE keyword, I decided to put inside the usage this note.

re #6962
Merge branch 'feature/6962_move_wide_angle_new_api' into develop