math/Imath: bugfix/docfix update to v3.1.5

• [#244] Fix 32-bit x86 build failure with 16c instructions
• [#241] Move numeric_limits specializations into half.h
• [#236] Change references to "master" branch to "main"

Full Changelog:
https://github.com/AcademySoftwareFoundation/Imath/releases/tag/v3.1.5

MFH skipped, we are just two days before 2022Q2.