[ADQL] Apply "ADQL-2.0 Erratum-3" :

The now optional coordinate system argument of `POINT`, `BOX`, `CIRCLE` and
`POLYGON` MUST be a _string literal_.

_See <https://wiki.ivoa.net/twiki/bin/view/IVOA/ADQL-2_0-Err-3>_

src/adql/parser/grammar/adqlGrammar200.jj

@@ -1234,7 +1234,7 @@ GeometryFunction GeometryFunction(): {Token fct=null, end; GeometryValue<Geometr
 }
 
 ADQLOperand CoordinateSystem(): { ADQLOperand coordSys=null;}{
-	coordSys=StringExpression()
+	coordSys=String()
 	{ return coordSys; }
 }
 

src/adql/parser/grammar/adqlGrammar201.jj

@@ -31,7 +31,7 @@
 * ParseException is thrown.
 *
 * Author:  Grégory Mantelet (CDS)
-* Version: 2.0 (04/2020)
+* Version: 2.0 (06/2020)
 */
 
 							/* ########### */
@@ -107,7 +107,7 @@ import adql.query.operand.function.geometry.GeometryFunction.GeometryValue;
  * @see ADQLParser
  *
  * @author Grégory Mantelet (CDS)
- * @version 2.0 (04/2020)
+ * @version 2.0 (06/2020)
  * @since 2.0
  */
 public class ADQLGrammar201 extends ADQLGrammarBase {
@@ -1345,7 +1345,7 @@ DistanceFunction DistanceFunction2(): { Token fct=null, end=null; DistanceFuncti
 }
 
 ADQLOperand CoordinateSystem(): { ADQLOperand coordSys=null;}{
-	coordSys=StringExpression()
+	coordSys=String()
 	{ return coordSys; }
 }