lots of updates to the readme to: improve scientific and technical clarity, reflect all recent changes to the CLI (especially the addition of prepare), reflect all recent changes to our installation, improve explanation of how to handle a few different kinds of data, and expand instructions on contributing and developing
* Scanpy engine now required
Without the --engine param we need to error if scanpy engine cannot be imported rather than waiting for all engines
* CLI options and help matches proposal
(but not all options hooked up yet)
* Flesh out top level args
* Move computation args to engine
* CLI input file (#374)
* Fix test command
(tests still won't work)
* Input is file instead of directory
- also renamed example file
* Csweaver/debug (#376)
* Respect debug flag for logging flask calls
* Add loading messages
* max categories (#377)
* Add max categories
* Rename max_categories to category_selection_limit
* ensure whole numbers
