isochrones

Currently the layer cannot be selected for low zoom levels.

Would be nice to tell a user when certain things happen while processing, e.g. clamping values to their allowed maximum, enabling destination_only roads in 2. pass, deprecating request parameters etc.

IMO it should be a top-level warnings array of strings which is empty by default and all modules can add their share during processing: probably implement the most obvious ones first and add o

Here's what I did

 [-0.0743414, 51.5015841],
 [-0.05273140000000039, 51.46430039999999],
 [-0.0531279, 51.4637