reusable utility functions

[jGaboardi]

Please review, but don't merge

This PR:

• helps get my feet back into things (been neglected this month)
• adds the utility functions mentioned in #22
• will resolve #22 when complete, but not yet
• if all looks good here, I'll apply these in the other notebooks where possible
• resolves #36

[jGaboardi]

• If this current state passes review, let's merge and I will pick back up in another PR
• @anastassiavybornova
  • I did some streamlining of osmnx.ipynb & usecases.ipynb (also reran to ensure success)
  • osmnx.ipynb
    • general tidying
    • moved video creation into plot loop
  • usecases.ipynb
    • I added a load_usecases() function in utils.py for easy access
    • Also streamlined the structure of the json
• @martinfleis
  • I renamed cityseer.ipynb to accommodate yours & mine upcoming exploration. So now we'll have:
    • cityseer_overview_gaboardi.ipynb
    • cityseer_parallel_gaboardi.ipynb (for my parallel edge exploration)
    • cityseer_<DESCRIPTION>_fleischmann.ipynb (for your exploration)

[jGaboardi]

@anastassiavybornova I'll wait to merge until I get you explicit approval since this touches your code. (no pressure/rush, though).