Update functions to handle 6-dims + add tests

[aTrotier]

Add more unit tests before changing the order of echo/channels in returns image

Might also be useful also for a major modification -> add trajectory to repetition dimension (and maybe others later if some applications need it) -> see PR discussion https://github.com/MagneticResonanceImaging/MRIReco.jl/pull/105#issuecomment-1262369519

[tknopp]

looks good to me

[aTrotier]

Do you see others functions that need a unit test ?

[tknopp]

There are certainly others, but it makes sense to make this step by step commit every improvement.