fix get_modelcube test
fix get_modelcube test

1. as the channel noise is now normal, the assert check now verifies
that the residual standard deviation is consistent with the true noise
variance.

2. removed unused sigma_threshold option

3. added mandatory conversion to km/s - otherwise CubeStack forces xarr
into GHz, thus making parcube xoff units invalid

keflavich/pyspeckit,allisony/pyspeckit,pyspeckit/pyspeckit,jpinedaf/pyspeckit,pyspeckit/pyspeckit,e-koch/pyspeckit,keflavich/pyspeckit,e-koch/pyspeckit,vlas-sokolov/pyspeckit,vlas-sokolov/pyspeckit,low-sky/pyspeckit,low-sky/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit