Document and validate rounding and snapping

We should inspect all rounding and snapping operations in OpenTimelineIO. We should write a policy on rounding and snapping. The code and policy should agree.

Unit tests should exist for identified operations that validate adherence to policy.

No files, entry points, or tests are named. Start by locating all rounding and snapping operations in OpenTimelineIO, then inventory them against a written rounding and snapping policy; done means the code follows that policy and unit tests validate each identified operation.

Written by the indexing model from the issue text.