Have the unifier supports deep types

[ngeiswei]

Motivation

The unifier https://github.com/opencog/atomspace/tree/master/opencog/unify, a crucial component of the URE https://github.com/opencog/atomspace/tree/master/opencog/rule-engine, must perform type intersection in the unification process.

Unfortunately type intersection has not been implemented for deep types, as a consequence deep types cannot be used in URE rules, or source or target variable declarations.

Task

To have the unifier support type intersection one would need to upgrade Unify::type_intersection to support deep types, so assuming we only bother about the inheritance cases of intersection, one would need to add deept type support to Unify::inherit https://github.com/opencog/atomspace/blob/master/opencog/unify/Unify.cc#L923.