Implement Inverse Distance Weighting Algorithm

[bransonf]

Leaving this as a note to self when I have more time to get back to areal.

Inverse distance weighting is an invaluable tool for creating raster surfaces from point samples. However, the implementations of this technique in R are poorly documented and rely on (highly problematic) Spatial* class objects. I've written, but not yet perfected, an sf implementation after spending hours trying to replicate sp methods.

It seems appropriate that an sf-based abstraction of IDW gets added as a function to areal

[bransonf]

Leaving another note-to-self.

The sp voronoi implementations mess up projections and the st_voronoi method seemingly does not work as anyone intends. (Including attribute preservation via this SO thread)

Voronoi is complementary to IDW in many ways, such that both are appropriate interpolation methods that could use coherent, user-friendly implementations in areal.