schemas FEATURE add schema getters from the context

Also check for the duplicities in the context when parsing a new schema.
diff --git a/tests/src/test_parser_yang.c b/tests/src/test_parser_yang.c
index db73f63..8f5085c 100644
--- a/tests/src/test_parser_yang.c
+++ b/tests/src/test_parser_yang.c
@@ -785,7 +785,7 @@
                  assert_string_equal("RFC7950", mod->ref));
     /* revision */
     TEST_GENERIC("revision 2018-10-12;}", mod->revs,
-                 assert_string_equal("2018-10-12", mod->revs[0].rev));
+                 assert_string_equal("2018-10-12", mod->revs[0].date));
     /* rpc */
     TEST_GENERIC("rpc test;}", mod->rpcs,
                  assert_string_equal("test", mod->rpcs[0].name));