Simplifier cleanup and improvements.

- make collapse/compact step work so we can collapse a NxN grid of cells
  into the parent if they all agree
- if the option to preserve geometries from single-feature cells is off,
  then we were already taking the lowest-level cell and making it into a
  block, but update to then collapse that cell up to the parent.
- for lowest-level cells that can't be compacted/blocked/etc., simplify
  their borders using JTS TopologyPreservingSimplifier.
- comments and code-cleanup