Write a tutorial for computing kinematic variables

[niksirbi]

Is your feature request related to a problem? Please describe.
After #3 is merged, we need a way to document the new features.

Describe the solution you'd like
We can make a new "example", similar to the one we already have, made with sphinx-gallery.
The example will load one of our datasets, and will walk users through computing the various kinematic variables, and going back-and-forth between cartesian and polar coordinate systems.

This should be also accompanied by adding the new functions to the API reference.

Additional context
We also need a "glossary" (see #69), where we give exact definitions of the variables we deal with and establish the nomenclature. But this should/may be a separate PR.

[lochhh]

Closing as completed in #135