Allow to correctly open bmlab files with repetitions of different dimensions

[raimund-schluessler]

Impose cannot open bmlab files with repetitions of different dimensions correctly, because it stores the meta data per file and has no concept of multiple repetitions in the same file.

Here

impose/impose/formats/fmt_bm_bmlab.py

Line 72 in ec7ea53

"shape": channels[chan[0]].shape,

we simply use the shape of the first repetition, but this fails if other repetitions have different shapes.