Maintain Python 2.6 compability

Comprehensive Dicts are not yet implemented in Python 2.6 (cf. PEP 274). In order to run scikit-rf with python 2.6 all comprehensive dict creations have been rewritten as dict creation of comprehensive lists.

Of course, comprehensive dicts have their advantages, but if somebody needs to run the code with python <= 2.6 this is the only way to do it. I'm not sure if you want to pull this in the development branch.