Add BackwardChainerGetLink

[ngeiswei]

Proposal

To slightly ease the usage of the URE and make it appear more as an extension of the pattern matcher, it would be nice to introduce

BackwardChainerGetLink

which would be a scope link that can be executed by the atomese interpreter and yield a similar output to GetLink.

Example

For example, executing the following

(cog-execute!
  (BackwardChainerGet
    (TypedVariable
      (Variable "$X")
      (Type "ConceptNode"))
    (Inheritance
      (Concept "A"))
      (Variable "$X")
    <rule-base-with-deduction-rule>))

on the KB described in issue opencog/atomspace#2472 would yield the following

(Set
  (Concept "B")
  (Concept "C")
  (Concept "D")
  (Concept "F"))