Move map generation functions from ipygis

[Rikuoja]

Is your feature request related to a problem? Please describe.
Currently, the Kepler map htmls are directly generated by https://github.com/gispoCoding/ipygis . That won't do if we want our own custom Kepler instance here (#49). Also, ipygis might be deprecated anyway. We want to be able to create the right kind of Kepler map config and the geodataframes for our Kepler instance.

Describe the solution you'd like
Move all the code required by generate_map to this repo. Instead of a Kepler map html, generate_map should return a Kepler config json with the geodataframe embedded, or alternatively, generate the config and the geodataframes and let Flask serve them to Kepler separately.

Also, tests are necessary. Currently, generate_map is far too fragile.