Rename `isochrones` to `reachability`

[chrstnbwnkl]

The isochrones method seems to be a bit of a misnomer, because in most cases, the isochrone endpoints can be used for isochrones and isodistances. Something like reachability could be a better suited name, what do you think? We just released 1.0.0 a couple months ago, so maybe something for down the road.

[nilsnolde]

Jep, agree, reachability is better! But definitely a major version sort of thing

[nilsnolde]

ah well, maybe not.. I guess we could support both for a while and add a deprecation label/notice. even though the Isochrone(s) objects are also "public" API, I wouldn't have a hard time renaming them to Reachabilit-ies/y..