Added a pressure tensor getter, which returns the pressure tensor via the second order moment (to be added in lbmpy with PR #83 ) Which is needed for the ESPResSo walberla bridge. To convert the second order moment to the prssure tensor the off diagonals needed to be multiplied by a factor of visc/(visc + 1/6) which currently is done in epresso, but should probably be done in walberla or lbmpy (I just was not sure where exactly to add it).