Overall package update.
Update lights anchoring in "Maya_Arnold_*" templates.
Ensure "Softimage_Arnold_*" templates compatibility with latest "SItoA" build.

Closes kelsolaar/sIBL_GUI#214.
Update session storage in "Maya_Arnold_*" templates.
Update "CHANGES.rst" file.
Update various utilities files.
Update ".gitignore" file.
Merge branch 'feature/v2.1.3' into develop