[Feature] Make background channel optional

[adamltyson]

Although the background channel is useful for the cell classification and registration steps, it is not always practical to double the data collected (and in the case of some fluorophores, a channel with only auto-fluorescence can be hard to obtain). It should be possible to supply one channel and:

• Only register this one channel
• Use 3D, rather than 4D input to the classification network (at the expense of poorer performance).