Fix bug in concatenate channels

[f-brinkmann]

Which issue(s) are closed by this pull request?

Closes #531

[pingelit]

Looks good to me. There might be a more elegant solution to #531 but this solve the problem from what I can tell.

[sikersten]

Thanks for noting the bug! What do you think of solving it explicitely by moving the if case to l. 196, e.g.:

if isinstance(signal, pf.FrequencyData):
    data = np.concatenate([s.freq for s in signals], axis=axis)
else:
    data = np.concatenate([s.time for s in signals], axis=axis)

[f-brinkmann]

Thanks for noting the bug! What do you think of solving it explicitely by moving the if case to l. 196, e.g.:

Did this as suggested