[ADQL] An ADQLTranslator now provides a list of all features it supports,

src/adql/parser/ADQLQueryFactory.java

src/adql/parser/feature/FeatureSet.java

src/adql/parser/feature/default_features.md

src/adql/parser/grammar/adqlGrammar201.jj

src/adql/query/operand/function/InUnitFunction.java

src/adql/translator/ADQLTranslator.java

src/adql/translator/JDBCTranslator.java

src/adql/translator/MySQLTranslator.java

src/adql/translator/PgSphereTranslator.java

src/adql/translator/PostgreSQLTranslator.java

src/adql/translator/SQLServerTranslator.java

test/adql/translator/TestJDBCTranslator.java

test/adql/translator/TestMySQLTranslator.java

test/adql/translator/TestPgSphereTranslator.java

test/adql/translator/TestPostgreSQLTranslator.java

test/adql/translator/TestSQLServerTranslator.java