Field-algebra based on Group / Ring symbolic vector module extension

Work in progress of abelian Group implementation which is extended to Field algebra.

julia> @ring xyz x y z

julia> x*y^2
xy²

julia> ans/x
y²

julia> x+y^2
x + y²

There are more undocumented features which will be explained once the design matures.

Fundamental principle of design is to construct Field algebra from an abelian Group algebra construction built on a vector space module.

Used in Similitude.jl and MeasureSystems.jl to implement dimensional group algebra.