Fix stacking when the spectral axis is decreasing

[e-koch]

@keflavich found a sign error in the stacking tools when the spectral axis is decreasing. All of the tests were only using increasing spectral axes.

I also added a test for stacking when the velocities are offset from 0.

[coveralls]

Coverage increased (+0.01%) to 75.781% when pulling 4ef4b5d on e-koch:more_stacking_tests into 33ffab8 on radio-astro-tools:master.