Add manifest and requirements to fix PipI shipping

Created by: gcarleo

This should (at least partially) fix most of the issues we have seen so far in #170 (closed). I have registered a new version

pip3 install netket

should work, provided that CMake is available and some MPI compiler is available. For some applications numpy is also needed, but users who try to run those applications and don't have numpy will see an error message which is clear enough.