Fix hidden merge conflict in setup.py

The install_requires parameter was added in both branches at different
locations.
Make cern a namespace package

If splitting the package (#30) and keeping the subpackages in the cern
namespace, we need cern to be a namespace package. Only then, multiple
independent modules can be installed into the same base package.
Move _checkCommand to cpymad.model

This function is only useful for this module. No need to keep it in a
separate location.