Trac #15171: Add a class for the value groups of discrete valuations

Currently there is no class to describe the value group of a discrete
valuation. This file adds a very simple class for that. This is in
preparation
of #15154 and following tickets.

URL: http://trac.sagemath.org/15171
Reported by: saraedum
Ticket author(s): Julian Rueth
Reviewer(s): David Roe