doc update
optimization
moved FileIterable to utils.dataPool2.files
moved factory for file iterators to file.iterator()
added .gz mimetype
added serialize and deserialize callback option:
        structure.serializeCallbacks = {"file": callbackFunc(value,
field)}
        structure.deserializeCallbacks = {"file": callbackFunc(value,
field)}
test update
timestamp as float
added File.iterator() and FileIterator class
render: ignore empty lists