start on #180: refer to MWPotential2014 in the introduction
test related to issue #190
test that qdf setup with IsochronePotential different from that in the actionAngleIsochrone instance fails (related to #190)
add _pot to actionAngleIsochrone and new logic to test whether the potential and the actionAngle's potential are the same in the quasiisothermaldf setup, fixes #190