mrcal 2.1 release notes

This is a very minor release, made primarily to facilitate the push to Debian. Updates:

• Lots of minor documentation improvements. These appear on the website
• Visualization functions use more legible colors to display the valid-intrinsics region
• Python library imports cv2 only where necessary, which dramatically speeds up all the tools where cv2 isn't used
• mrcal.r_from_R() works better with small rotations. Near-identity rotations now report correspondingly-small r, rather than 0

This is a very minor release, and is 99.9% compatible. The only incompatible update:

• mrcal.annotate_image__valid_intrinsics_region() draws in green by default for consistency with other functions