Add option to include grid data in isochrone pbf output

[chrstnbwnkl]

similar to #4594 but for the more analytical use cases where visualization of the grid is not a priority. Could be achieved by adding another request parameter that when used in combination with format=pbf writes the grid (and additionally needed information like cell size and top left corner coordinates) to the isochrone pbf message and skips the contouring.

[nilsnolde]

Can be done, but I'd say very low prio (in my book at least).