Add optional unions with bind query clause

[syphax-bouazzouni]

Used for ncbo/goo#124
To have this sort of query

OPTIONAL {
   { ?id <attribute_1_uri> ?attributeObject . BIND("var_1" as ?attributeProperty) } 
  UNION	
    ....
    UNION
   { ?attributeObject <attribute_j_uri> ?id . BIND( "var_j" as ?attributeProperty)} # if attribute_j_uri is an inverse attribute 
   UNION 
     ....
   { ?id <attribute_n_uri> ?attributeObject . BIND("var_n" as ?attributeProperty) } 
}