Remove the OpenEXR dependency and instead add Imath 3

[yuki-koyama]

Alembic is now able to be built directly with Imath 3 instead of OpenEXR.

• https://github.com/alembic/alembic
• https://github.com/AcademySoftwareFoundation/Imath

Imath 3 is more lightweight than OpenEXR and so it is desirable to use it.