Refactoring of the BandMathX documentation, and fix OpticalCalibration documentation
Summary
This merge request replaces the AboutBandMathX C++ section by a recipe: Operations on images with BandMath and BandMathX
The BandMath part of the ImageProcessing recipe has been moved to the new recipe.
This merge request also fix a bug in the OpticalCalibration application documentation (a newline was needed before a bulleted list). See issue #1956 (closed).
Rationale
See issue #1904 (closed)
Copyright
The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.
Check before merging:
- All discussions are resolved
 - At least 2 
👍 votes from core developers, no👎 vote. - The feature branch is (reasonably) up-to-date with the base branch
 - Dashboard is green
 - Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
 - Optionally, run 
git diff develop... -U0 --no-color | clang-format-diff.py -p1 -ion latest changes and commit 
Edited  by Cédric Traizet