distinct isochrones for multiple points

When requesting isochrones of more than one center point, they may overlap.
Most notably, areas with a low timecost may be "shadowed" by areas with a higher cost from a different point. This makes the data unusable for further analysis.

To solve this I manually ran the following process, but it would be better if this was automated and enabled by default:

- split the layer into multiple layers grouped by `AA_MIN` column
- calculate geometric difference between each resulting layer, resulting in non-overlapping polygons
- dissolve each resulting layer to merge adjacent polygons
- merge those layers back into one

**edit**: I believe this is what was requested in #26 as well.