Fix pauli strings

Created by: PhilipVinc

Closes #580 (closed)

Since we are not testing PauliStrings with anything really, I had missed that they needed methods to work with MetropolisHamiltonian, as well as is_hermitian defined to dispatch to the right gradient method.

This is still missing tests.

(Also adds a short description line to setup.py which is now required for recipes on conda...)