bootstrap the BaseFrame unit tests.
extract the I/O layer and put in a separate module.

--HG--
rename : tests/test_format_iosource.py => tests/test_iobridge.py
merge the autogenerated enums in a single source module.

--HG--
rename : pyrana/pixelfmt.py => pyrana/ffenums.py
shorter repr()s. Still WIP.

Not satisfied neither with former nor with the current.
cleanup, docs and pylint fixes.
a bit more testing
Pull from HG.