Added option to not serialise virtual fields to both json and msgpack codecs, added filter for composite fields to ResourceOptions (the _meta attr)
Fix bug in ApplyMapping helper, was not returning a value
Fix bug in composite fields list, where f.or is a type rather than an instance
Added item_iter_from_object  to composite fields. This method provides a key/value iterator for any composite field.

This feature is used by the Transverse tools to allow them to discover sub resources. For ListOf fields the key is the index, for DictAs the key None and the iterator always returns a single item