Add description of characterizing section.
Describe crossing parity vector method for determining homology.

This is just a copy-paste plus necessarily cleanup to make it readable.
Describe the signature method for determining homology.
Standardize statement of characterization lemmas.
Describe first method of characterizing homology as using arcs in introductory description.
Rewrite description of crossing parity vectors using language from the homology cover paper.
Rephrase reminder that both methods of characterizing homology use tree-cotree decompositions.
Add warning about running Henzinger et al. after contracting the boundaries.