Fixed classy finding

[ThomasTram]

resolves #371

[schoeneberg]

Just wanted to add that the newest classy can also install itself into the user's site-packages, so providing an explicit cosmo path is very weird for that. The solution should be as simple as allowing for
data.path['cosmo'] = 'detect'
or something like that, and then simply trying to import classy without verifying the location (maybe printing a warning/info to the user which classy is finally being picked). That'd be much nicer than the user having to list their site-packages directory inside of the data.path['cosmo'].