Add support for top+small , bottom+small

[samaloney]

I'm not sure if there is really a use case but it occurred to me when working with @DanRyanIrish on TCDSolar/stixpy#148 that one could possibly want to use either the top, or bottom rows of pixels and the small pixel e.g. when the source is outside the full illuminate FOV. I realise the phase correction would be the same as big+small but at the moment would be hard to do. What do you think @paolomassa?

[paolomassa]

I need to do the math for that case, I'll get back to you later on this!