rpy2 no longer supports python2 for which CERES is made

[regislachaume]

After one installs these dependencies:

• Rlib (package depends on distro, may need to symlink manually at /usr/lib/Rlib.so),
• libreadline (package depends on distro, needs the dev/devel one),

one needs to install an old enough rpy2 version:

• pip install -Iv rpy2==2.5