relax SVN archive

Much of the Python 2 verses 3 compatibility, as well as different Python 2 version compatibility and
different Python 3 version compatibility, code has been shifted into the compat module. The
different parts of relax now import from the compat module for modules/packages with different
import semantics for different Python versions.

In addition the different handling of the bz2 and gzip module for reading and writing files has been
shifted from 'relax_io' into 'compat'.