V2d serialization fix

[rogernelson]

Fixes #1855

Summarize your change.

This is a bug fix where V2d was getting serialized into two V2ds inside a Box2d. The problem was that there was no write_value function in the serialization for V2d (I guess it was assumed a V2d would always be inside a Box2d). Box2d has an implicit ctor taking a V2d, which implicitly converted the V2d to a Box2d to match on the Box2d write_value method.

Notes:
make test required me to run make doc-model-update which added some whitespace changes to otio-serialized-schema.md
make lint changed some whitespacing in the v2d test

Reference associated tests.

Two new tests added to test_v2d.py

• Check the schema is correct when serializing to json
• Round trip serialization/deserialization ensures we get the original value back

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rogernelson / name: Roger Nelson (afc83cd)

[ssteinbach]

I think the changes to otio-serialized-schema.md might be a mistake (or at least, I want to double check that they aren't). Otherwise LGTM.