[ADQL] ADQLParser is no longer a Java Interface and ADQLParserFactory does not

exist anymore.

ADQLParser is now a mix between the former ADQLParserFactory and ADQLParser2xx.
All ADQLParser2xx resulting from the compilation of the JavaCC files are
now named ADQLGrammar2xx and implement the new interface ADQLGrammar. The JavaCC
parsers are no longer used directly. This is the role of ADQLParser to simplify
the parser usage and to allow an easy switch between different grammar versions.

Besides, for more clarity in the class organisation, all generated parser
classes have been moved into the package `adql.parser.grammar`.

test/adql/parser/TestIdentifierItem.java

 package adql.parser;
 
-import static adql.parser.ADQLParser201Constants.DELIMITED_IDENTIFIER;
-import static adql.parser.ADQLParser201Constants.REGULAR_IDENTIFIER_CANDIDATE;
+import static adql.parser.grammar.ADQLGrammar201Constants.DELIMITED_IDENTIFIER;
+import static adql.parser.grammar.ADQLGrammar201Constants.REGULAR_IDENTIFIER_CANDIDATE;
 import static org.junit.Assert.assertEquals;
 
 import org.junit.Before;
@@ -9,6 +9,7 @@ import org.junit.BeforeClass;
 import org.junit.Test;
 
 import adql.parser.IdentifierItems.IdentifierItem;
+import adql.parser.grammar.Token;
 
 public class TestIdentifierItem {