Adding combine turf method

[langsmith]

This pr ports the combine method to Turf for Java. This method takes a FeatureCollection of Point, LineString, or Polygon features and returns a Geometry. The returned Geometry can be used to create a MultiPoint, MultiLineString, or MultiPolygon feature.

http://turfjs.org/docs/#combine

https://github.com/Turfjs/turf/blob/master/packages/turf-combine/index.ts

combine(FeatureCollection featureCollection) is the new method added to the TurfConversion class.

[langsmith]

The returned Geometry can be used to create a MultiPoint, MultiLineString, or MultiPolygon feature.

For example,

 MultiLineString newMultiLineStringObject = (MultiLineString) TurfConversion.combine(lineStringFeatureCollection);

[langsmith]

Return type should be a FeatureCollection

Fixed

[langsmith]

Ok, FeatureCollection is now returned in all scenarios and I pushed some refactoring which checks that all geometries are of the same type. If not,

mapbox-java/services-turf/src/test/java/com/mapbox/turf/TurfConversionTest.java

Line 185 in c3524ee

thrown.expectMessage(startsWith("Your FeatureCollection must be of all of the same geometry type."));

is thrown.

[langsmith]

👉👉@tobrun

[langsmith]

Ok @tobrun . Pushed some changes and responded to comments above. Would love another round of eyes when you get a chance 🙇

[langsmith]

👉👉 @tobrun 😬

[tobrun]

A single FeatureCollection is the parameter. The FeatureCollection has to have Point and/or MultiPoint, LineString and/or MultiLineString, or Polygon and/or MultiPolygon. No mixing Point-based geometries with Polygon-based, etc.

The heterogeneous test showcase that is should be possible to combine both Point with Polygon. To get the proper implementation on the java side, I should try fully porting the testsuite on the js implementation. This helps to verify assumptions you are making about the code.

[langsmith]

The heterogeneous test showcase that is should be possible to combine both Point with Polygon. To get the proper implementation on the java side, I should try fully porting the testsuite on the js implementation. This helps to verify assumptions you are making about the code.

I'd missed the inclusion of the Point at https://github.com/Turfjs/turf/blob/master/packages/turf-combine/test.js#L188 🔬 . Confirmed by getting more clarification at Turfjs/turf#1756.

Refactored the method implementation and tests so that heterogenous combinations are allowed. Rebased. The order of my tests in this pr now match the test order in https://github.com/Turfjs/turf/blob/master/packages/turf-combine/test.js. While the coordinates differ, my tests check the same situations as in https://github.com/Turfjs/turf/blob/master/packages/turf-combine/test.js (e.g. Point & Point, LineString & MultiLineString, etc). I also added combinePointAndMultiPolygonAndLineStringGeometry, combinePointAndLineStringGeometry, and combine_featureCollectionSizeCheck.

Would love another round of 👀 when ya'll get a chance.