make kernels public.

I'm not sure if we want to do this, but something like this was needed to address: urschrei/polylabel-rs#6

geo/src/algorithm/mod.rs

@@ -1,5 +1,5 @@
 /// Kernels to compute various predicates
-pub(crate) mod kernels;
+pub mod kernels;
 
 /// Calculate the area of the surface of a `Geometry`.
 pub mod area;