psoct.multi_slice: do not require a channel dimension

[balbasty]

In the multi_slice OCT converter, it is assumed that the onput file contains a channel dimension.

It is the case in some of our data, but not in the minnesota data, and potentially not in some of our (non orientation) data as well.

It would be good to have a flag to handle the presence or absence of a channel dimension.

@calvinchai

[balbasty]

Weirdly, the single_volume script does not expect a channel dimension, but expects the data to be 3D. Maybe this one should also have flags to handles the 2D/3D and channel/no-channel cases?

[balbasty]

Fixed by #27