stream_sign set to +/- 1 depending on the sign of the parallel streaming term. NB: stream_sign = -1 corresponds to positive advection velocity only need to consider ia=1, iz=0 and is=1 because alpha, z and species dependences do not lead to change in sign of the streaming pre-factor get gradpar centred in zed for negative vpa (affects upwinding) get gradpar centred in zed for positive vpa (affects upwinding)