Add support for derived properties

[davidpaquin]

See the README changes for an example.

Some things to note:

• Validation for links was moved from defineLink.ts to convertLink.ts to solve some circular dependency problems between derived datasources, links, and objects
• There is a break in defineObject where instead of a datasource? field, there is now a datasources field that is an array of datasource definitions. A default dataset source will still be produced if a "base datasource" is not specified (dataset, stream, RV)
• There's still some validation that we could do, like making sure that the types of the current and foreign properties are compatible (e.g. right now we allow doubles to be collected into a list of integers, which we probably shouldn't?)

[aep000]

Lets ship it