Test for virtual spectrum required

[unoebauer]

Issue #445 clearly demonstrated that we need to test explicitly the correct determination of the virtual spectrum. We could simply add a test_virtual_spectrum method to test_tardis_full.py, which does essentially the same as test_spectrum but for the virtual spectrum.

This would imply that we do another bitwise spectral comparison. I guess this is our best option until the statistics project has matured enough....Other ideas, @orbitfold, @wkerzendorf, @ssim, @chvogl?