Generalize feature collection param in nearest_point?

[deeplook]

It would be much more convenient to allow arbitrary feature collections in turfpy.measurements.nearest_point(pt, fc). Right now this has to be a list of points, which the user will likely often have to create from an existing feature collection. Such a change might result in a more general API than the one originally offered by turf.js. Which needs to be explained, etc.