Rjf/isochrone simplification

[robfitzgerald]

this PR adds new isochrone drawing features, adding an optional isochrone simplification algorithm to run for ConcaveHull isochrones.

ConcaveHull built from points along linestrings at some reasonable stride can produce quality representations for many travel modes:

However, this algorithm can produce many vertices leading to substantial disk space requirements.

Rust Geo's Simplify algorithm accepts a tolerance and will combine vertices within that tolerance. Simplification can greatly reduce the number of redundant vertices. here are some results of exploring simplification on a test polygon:

	tolerance_latlon	tolerance_meters	coords
0	0	0	1497
1	0.00135	14.9999	41
2	0.0027	29.9997	22
3	0.0054	59.9994	19

Configuration

To add simplification, use the optional simplify_epsilon_latlon parameter:

isochrone_algorithm = { type = "concave_hull", concavity = 2.0, simplify_epsilon_latlon = 0.00135 }