Better support for custom `PositionProperty` implementations

I wrote my own `PositionProperty` class. (It's very simple, it just wraps a collection of timestamped locations and gives the most recent one at or before the requested time -- kind of like `TimeIntervalCollectionPositionProperty` but less complex.)

I looked at the code for most of the other implementations, and in all cases `getValue` is implemented as a call to `getValueInReferenceFrame`, which calls `PositionProperty.convertToReferenceFrame`. This function is marked `@private` in the JSDoc, and not included in the TypeScript typings.

My first suggestion would be to remove this `@private` decoration and add type information to the method. Implementing classes generally want to use this method to translate to the correct reference frame after determining the (probably fixed-frame) "initial" value.

My second suggestion is to convert the `PositionProperty` class-like to a real inheritable (ES6) base class, [using abstract-like conventions](https://stackoverflow.com/a/30560792/26286), and refactor common implementations up into it. I think you've been avoiding real classes, which is fine, but it would be nice to reduce the workload for subclasses.

Start by reading PositionProperty and the getValue, getValueInReferenceFrame, and convertToReferenceFrame entry points described in the issue, along with the existing implementations that call them. Determine whether the intended direction is exposing typed conversion support or introducing an inheritable base class; done requires an agreed design and corresponding support for custom PositionProperty subclasses.

Written by the indexing model from the issue text.