Support for importing to Date field type

In this patch:
- new function gc_gc_process_default_field() for handling GC fields (default case.)
- additional logic for date and datetime fields

oops, left a debugging line.

Translation is identified and initialized first thing in the new helper function:

+function gc_gc_process_default_field(NodeInterface &$entity, $local_field_name, $is_translatable, $language, $field) {
+
+  $value = $field->value;
+  $target = &$entity;
+  if ($is_translatable) {
+    $target = $entity->getTranslation($language);
+  }

It's a different, more DRY approach than elsewhere in the module.