Store processed (but not reconstructed) data in a different place in the nexus file

[samtygier-stfc]

Desired Behaviour

When saving a dataset to a Nexus file, we currently store back into NXtomo/NXinstrument/NXdetector. When discussing with the nexus devs if we could floating point data here is was pointed out that its not good to put processed data back here because the location is intended for raw data.

Instead we should store it as an NXdata entry (we can use the same attributes like image key). We could also use NXprocess to record that the data is processed.

We will need to make sure loading looks in both places.

[samtygier-stfc]

Closed in #1734