Add instructing error messages in the case of node binding inequalities inside HAVING clauses

[rogerlucena]

From now on, comparisons between a node binding and a node literal inside HAVING clauses can only be equalities, such as in:

SELECT ?s, ?height
FROM ?test
WHERE {
    ?s "height_cm"@[] ?height
}
HAVING ?s = /u<bob>;

Note that node inequalities are not allowed anymore because they may raise confusion with regards to what is indeed being compared (only the ID, only the TYPE, or both together).

In the case of inequalities an error is prompted orienting the user to extract bindings with the ID/TYPE keywords and to use these bindings to proceed with the comparisons in a clearer way.