New quickstart and examples

[iancze]

v0.3 plan is

• write an end-to-end MPoL-dev/examples for SGD with the mock dataset.
• write an end-to-end example using the GriddedDataset and GriddedNet.
• cut out useful snippets into a "Getting Started" tutorial to introduces key MPoL concepts, assuming familiarity with PyTorch idioms. Goal is not to duplicate examples, but to showcase functionality. Might be redundant with the end-to-end examples, but we won't know until that is done.

Supersedes #134