Refactoring on the operators: negation is now being handled globally

Other minors changes were also made, including adding the prefix `m_'
to all the members of the class.

src/operators/geo_lookup.cc

@@ -96,8 +96,8 @@ bool GeoLookup::evaluate(Transaction *trans, const std::string &exp) {
 GeoLookup::GeoLookup(std::string op, std::string param,
 bool negation)
     : Operator() {
-    this->op = op;
-    this->param = param;
+    this->m_op = op;
+    this->m_param = param;
 }
 
 }  // namespace operators