Rate should be int/int not float

Currently RationalTime is represented as a float value and a float rate. Unfortunately there are cases where floating point arithmetic leads to precision issues, rounding and undesirable drift.

Some other systems represent rate as an int/int rational number. For example 30000/1001 vs 29.97.

We would like to experiment with this alternate representation of rate for OTIO. In order to do so, we propose two concrete steps:
1. Write a unit test that clearly demonstrates a flaw in using a floating point rate.
2. Create two different time classes (perhaps RationalTime and FractionalTime?) that can be used interchangeably in OTIO, but with different representations of rate.

By doing this we can ensure that we won't break anything by making this switch and we can clearly explain to everyone (ourselves included) the difference between the two and the reasons for choosing one over the other.

Comments?

Start by reading the existing RationalTime implementation and its unit tests, then identify how rate is represented and used across OTIO. Add a test demonstrating floating-point precision or drift, and evaluate interchangeable RationalTime and FractionalTime representations without breaking existing behavior.

Written by the indexing model from the issue text.