@turf/concave improvements

[stebogit]

• set maxEdge parameter optional and by default to Infinity.
  A common expected output of a concave function (like concaveman) is always to return a single concave hull Polygon. @turf/concave has the capability to return a multipolygon (with holes! 👍 ), but this should not be the default behaviour.

• remove Error output if there is no hull to return, it should return an empty Feature (helpers.feature(null)). (done here).
  Currently the Error "too few polygons found to compute concave hull" is likely due to a small maxEdge. This shouldn't be considered an error, because an error should mean a misuse of the module (which isn't the case here).

Ref. #907.

[DenisCarriere]

👍

[stebogit]

@DenisCarriere are we sure we should return a helpers.feature(null) if no concave hull is possible?
I'd say it should return null, for the same reasons why we have now null from @turf/difference and @turf/intersect

[DenisCarriere]

@stebogit 👍 Agreed, I forgot about this module having feature(null).

If there's any other modules returning a single Feature with Null geometry, it should be changed.

[DenisCarriere]

Also seems like the @turf/great-circle is doing the same:

turf/packages/turf-great-circle/arc.js

Lines 36 to 44 in fa7acb5

	Arc.prototype.json = function () {
	if (this.geometries.length <= 0) {
	return {'geometry': { 'type': 'LineString', 'coordinates': null },
	'type': 'Feature', 'properties': this.properties
	};
	} else if (this.geometries.length === 1) {
	return {'geometry': { 'type': 'LineString', 'coordinates': this.geometries[0].coords },
	'type': 'Feature', 'properties': this.properties
	};

[stebogit]

Implemented with #1020 and acbe77d