Quick Start Script and Readme

[WilsonGregory]

Changes

• this looks like a large PR, but its actually the same content 3 times because I put the quick start entirely in the README, then I put it in a python script, and then I put it in a python notebook as well
• make a quick start guide to create and initialize a MarkerMap, run it on cite-seq, and evaluate the performance
• make a parse_adata function that converts an adata object into X,y, and encoder to reduce code repetition. I will update the other notebooks later

Tests

• run the quickstart notebook
• run the quickstart script
• inspect the README

Doc Changes

• add a quick start section to the README