Add graticule data generator.

[jheer]

Changes:

• Update data types to include both data sources and data generators.
• Add generator for graticule GeoJSON data.
• Add tests / examples for graticule generation.

Close #3337.

[jheer]

@domoritz When you get the chance, please feel free to check my TypeScript. For example, there is some redundancy between GraticuleParams in the Data definitions, and the vega-schema VgGraticuleTransform. Not sure if there is a preferred way to model these bits.

[kanitw]

I haven't got the time to review thoroughly, but I response to the question about TS. :)

[jheer]

@domoritz or @kanitw, can I get a review on this PR? Thanks!

[kanitw]

Yes, but FYI I'll traveling a lot this and next week though.
(I mostly get time to hack on the plane, but I can't do code review without access to Github.)

[jheer]

Totally understand, but I was planning to write curriculum materials that would build on these and related geo updates within the next week. So that would require both a review as well as a 3.2 release. I'm hoping this is possible.

[domoritz]

If I have internet on the bus to Vancouver, I will do the review today.

[domoritz]

Looks good. Feel free to merge but either 1) add documentation or 2) create an issue to add documentation later.

[kanitw]

If we want a release, we should have a documentation, so we can point the release note to the documentation?

[jheer]

Yes, I plan to add docs very soon, along with a few additional generators (sphere and sequence).