Spectrogram.__str__ hard-codes order of axis types

[DanRyanIrish]

This line needs to prints out the axis types in the order which they are in the dat cube. At the moment it assumes the order is always (Slit steps, Slit height, Spectral).

[kakirastern]

Looks like the solution is not immediately obvious... Guess will need to test the code with some spectrograms to find out the pattern or to get a sense.