Feature reducers (#49)

* Temporarily type geomEach callback to GeometryType

* Add geomReduce function and tests

* propReduce function

* WIP - featureReduce()

* WIP

* added examples and documentations

* flattenReduce WIP

* WIP

* coordReduce

* documentation

* add static types

* merge readme

* prepare new merge

* generic types for all reducer functions

* TurfJS compatible solution with type checks

* add cloning

* fixed types passed to callbacks, propReduce test

* flattenReduce and featureReduce test

* coordReduce

* minor type conv. num to int

Co-authored-by: Arman Torkzaban <[email protected]>
Co-authored-by: Lukas Himsel <[email protected]>