Generate classes with: 

generateDS.py --subclass-suffix="" -f --super="cff" -o ../cff.py -s ../cfflib.py -a "xsd:" --namespacedef='xmlns:cml="http://www.connectomics.org/cff-2" xmlns:dcterms="http://purl.org/dc/terms/"' connectome.xsd

You need generateDS to create the object model from XML Schema:
http://www.rexx.com/~dkuhlman/generateDS.html

Convention
----------
.src -> refering to relative path in filesystem (fs) or zip file (iszip = True)
.tmpsrc -> refering to absolute path in fs or tempfile. a tempfile is created when .content != None.
schema to convert absolute file names to relative: CXXX / unique_name.fileformatending

In cfflib
---------
* Implement save for iszip == False
* how to save imagestack? save hdf5? save trackvis?
* test for all but network the saving mechanism for loaded

* [METHOD] validate file existence
* [METHOD] validate file type