Fitting velocity gradient

Inside moments_classic, we now fit a (non-)weighted LSQ plane to the x,v,v,t values to determine the velocity gradient (props.velgrad [km/s/pc]), position angle (props.velposang [rad]) of the velocity gradient axis, and the velocity dispersion (props.veldisp [km/s]) after correction of the velocity gradient across the cloud. The LSQ fitting requires two IDL functions: "planefit.pro" and "rob_mapfit.pro" by H.T.Freudenreich, which I added to the utils/ directory. The code always returns finite values, the user needs to make sure if those values are sensible with respect to the velocity resolution of the data cube. Cheers, Andreas