Add Isochrone tool

[frasercl]

Adds a new Isochrone tool for analyzing and planning traverses. Features:

• Render isochrones as raster layers showing color-ramped accumulated cost out to a set maximum
• Hover over isochrones to show least-cost paths and minimum cost from start point to hovered point
• Separated cost model interface allows easy extension and switching models on-the-fly
• Add, remove, and separately edit multiple isochrones
• Configurable color ramps, color steps, opacity, resolution, data sources

[tariqksoliman]

So she's restricting her movement to not make the king feel inferior?

[frasercl]

Don't know a lot about chess haha - I believe that's a term I saw somewhere else, and I didn't think about it too much. Would probably more correctly be called kingMoves.

[tariqksoliman]

queen is fine; the comment and 8 elements explain it pretty easily.

[frasercl]

And those moves can compose to queen moves, so it's not as if it's completely inaccurate.

[tariqksoliman]

LGTM and thanks for pulling this in!