New makeChildrenAppender function and Node factory refactoring

The makeChildAppender function is used for adding a node of a
type with maxOccurs=1. For adding nodes of a type with
maxOccurs>1, the new makeChildrenAppender function was added.

With this new function, it turned out that more convenience for
creating node factories is required. The makeChildNodeFactory
function was renamed to makeSimpleNodeFactory, and it now can
create node factories where not only the namespace, but also the
node name can be fixed.
Added rte serialization for ol.format.GPX