fix a bug revealed by Charlie Figura in which 'dxarr' was not being set properly after cropping/slicing an X-array.  I still don't fully understand why this happened at all, but the workaround now works.  A test is probably needed...
fix a bug revealed by Charlie Figura in which 'dxarr' was not being set
properly after cropping/slicing an X-array.  I still don't fully
understand why this happened at all, but the workaround now works.  A
test is probably needed...

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