Write geometric half of software overview
Write geometric half of software overview

We split the discussion of the internals of Abelfunctions into an
"algebraic half" and a "geometric half". We write a draft of the
description of the geometric half of the documentation.

cswiercz/abelfunctions,cswiercz/abelfunctions,cswiercz/abelfunctions,abelfunctions/abelfunctions,abelfunctions/abelfunctions,abelfunctions/abelfunctions