Implement time-variant transforms

[hdamron17]

Similarly to TF2, we want to add a second key for each transform which allows transforms to have different values over time. Some ideas for the implementation details:

• We would still have the option to getLatestTransform which takes the most recent version in addition to a time-dependent version.
• For specific times, we would take the nearest past transform and the nearest future transform then interpolate between the two. Possibly there is an optional threshold for how far away the transform can be.
• We need to add exceptions for requests which are connected at some point but not at the time requested.
• It may be helpful to allow range-based requests (e.g. less than 20 seconds ago)
• We would need convenient deletion methods to help people not cause memory leaks. Also the option to automatically delete transforms which are a certain amount older than the newest frame (this is how TF2 does it).
• Although caching probably wouldn't be very useful for older transforms, we could have it for the latest transform.