Split out Unify into it's own git repo

[linas]

Unify is now in https://github.com/opencog/unify
Make it into a proper dependency for the URE.

Splitting out Unify enables it to be developed on it's own.

General plan for Unify: Create a new Atomese UnifyLink

As far as I can tell, (Unify A B) is equivalent to (Get (Identical A B)) In greater detail,

(Unify
    (Inheritance  (Concept "A") (Variable "$Y"))
    (Inheritance (Variable "$X") (Concept "B")))

is equivalent to the more verbose

(Get
    (VariableList
       (Variable "$X") (Variable "$Y"))
    (Identical
         (Inheritance  (Concept "A") (Variable "$Y"))
         (Inheritance (Variable "$X") (Concept "B")))))

Both will return

(Set
    (List (Concept "A") (Concept "B")))

Differences are unclear, I'm hoping @ngeiswei will help clarify over time (back-burner project)

[ngeiswei]

Oh, I didn't know (Get (Indentical A B)) acted as a unifier. If so, all that unify code might just be redundant. There is a plethora of tests under tests/unify, maybe some important differences can be found.

[linas]

I now understand that the unifier, and IdenticalLink are similar, but not the same. The Unifier handles more difficult cases. This is reviewed in opencog/unify#1