Improvements to allow for pip install

- renamed script to setup.py
- Use "make pip" to copy required files in this dir for pip
- Use try/except when importing numpy and cython so as to notify the user
- renamed env vars to plumed_xxx so that there is no risk of clashes

Plumed can now also be installed using

cd python
make pip
pip install --user .

make pip is just needed to bring version information and Plumed.h
in this directory. Might be solved differently in MacPorts / Pypi.