Isochrone Request Optimization

Currently, the [plans](https://account.heigit.org/info/plans) and [restrictions](https://openrouteservice.org/restrictions/) of the public openrouteservice API on the standard allow for 500 isochrone requests and at most 5 locations per request.

Thus, 2500 isochrones (not counting intervals) can be calculated.

For "large" isochrones (lots of intervals, large scale) sending one location per request seems appropriate.

For "small" isochrones (few intervals, small scale), we could send up to 5 locations per request.

I propose an "optimize requests" button in the `isochrone from point layer` processing algorithm.
This should have a bit of logic, looking into the request, and decide on an execution strategy for processing isochrones.
The logic should be explained in the docs, then.