cpnest broken

I managed to break our implementation of cpnest when reformatting the samplers. I thought I checked it, but it looks like I didn't check properly.

There's an issue with our files having the same name as the modules being imported, i.e., import cpnest tries to import our module called cpnest, not the actual cpnest.

tupak/core/sampler/cpnest.py", line 54, in _run_external_sampler
    import cpnest.model
ImportError: No module named model

EDIT: Confusingly, I can run with cpnest on my laptop but not on CIT:pcdev2.