Read Ltotal from nexus file rather than from detector data array in tof workflow #183
Description
To allow for efficient live view of streaming data, we don't want to re-load positions every time a new chunk of events comes in.
After #180 is merged, the tof workflow will extract the ltotal from the detector data.
We probably want to read that from the file once, and only have to operate on the events.
In practice, this is quite simple. It means that Ltotal shouldn't come from RawData, but just be a free parameter.
