Multiple datasets support

[jcc37-zz]

Branched off of #44

• Add dataset types and interfaces to expose general APIs to retrieve data from different sources
• Refactor PreferenceSerice and support "select" type preference item
• Modify DataService and Preference to support switching between different sets of dataset preference
• Merge Add tests for d3 #26 to Add tests for services #30 and update old tests for the new code structure and data flow
• Add new tests for component and dataset

What's next

• Refactor UserWhiteNoiseDataset
  • Create util functions to create dataset query function with lazy-query and caches
  • Move common category generator to utils, like nthDay in DataCube
• Refactor XYChartD3
  • Support data points with different x and y types for other chart components

[Rhombus33]

You don't need to do this in this PR, but generally it's nice to have a small documentation for exported methods, services or any significant chunk of code so that someone can get the gist of it without reading through all the code

[jcc37-zz]

It raises another design question here. Actually Config, configMeta, and create are three necessary exports/properties for all dataset module/object, it is nice to have a abstract parent interface or class for all datasets to extend. However, they would be all static and immutable properties and I don't want developers to create instance of dataset if I make them as classes, so I make them modules when adding dataset support.
The bad things of this design are that it is hard to build mock datasets for testing and do the type inference on the map of dataset modules:

guide-doge/src/datasets/index.ts

Line 4 in 0b20324

export const datasets = {

I would like to refactor it if you have any good ideas.

[64json]

Off-topic but it would be great if TypeScript supports a module to implement an interface:

export module implements AbstractDataset;

Then all the members defined has to be implemented and exported, so people don't need to have an extra wrapper to make it type-safe.

EDIT: found similar discussions on microsoft/TypeScript#420 and microsoft/TypeScript#38511.