Handle multitomo for 'online'

On the long term ID16b would like to treat multi-tomo ASAP.

Multi-tomo is taking projections over several turns around the sample. We would like to generate NXtomos ASAP (so when n projections are acquired - usually one half a turn or a turn is done).

Then when the sequence is finished we want to 'regenerate' those files (as some dark / flat) might be acquired at the end and we want to have the 'complete' NXtomos.