SuReal: changed similiar-atoms

Not sure if this change is needed or not.  The previous behaviour might have been intentional?

Previously, similar-atoms thinks
```
(InheritanceLink (ConceptNode "dog") (ConceptNode "mad"))
```
and
```
(InheritanceLink (ConceptNode "yellow") (ConceptNode "dog"))
```
are similar, even though "dog" is at a different position.   This caused

```
(r2l "The mad dog hates the tree.")
(sureal (SetLink (InheritanceLink (ConceptNode "yellow") (ConceptNode "dog")))
```
to return "The dog yellow hates the tree."

Also changed `get-mapping` to use similar-atoms so the same criteria is used throughout.
Sure, I will try and see how to put it in :)