Clean up coding style in cellmlmodel.py.

Reorder imports as per http://www.python.org/dev/peps/pep-0008/#imports
Use np.recarray instead of separate import.
Removed unnecessary alias for cgp.physmod as cellmlmodels.
Generate _cellml2py in user directory to allow root install of cgptoolbox.

sys.path is extended with $HOME/_cgptoolbox at runtime.
Fix generation of benchmark code in Cellmlmodel.makebench().