i15-1: Eiger Data & NeXus File Writing

During testing by @keeble , a few issues were noted:

- [x] The eiger data is being written as unsigned when it should be signed (the masked value should come through as negative, not 4294967295 which is 2**32 - 1)
- [x] The eiger data have at least some frames in which don't look right. This seems very consistent, the frames showing something currently have a max of 2**32 - 1. Seems to be pretty consistently [good, bad, bad, good, good, ... etc.].
- [x] The path to the eiger data in the nexus file should probably be /entry/instrument/fastcs_eiger/data , whereas currently it's /entry/instrument/fastcs_eiger/fastcs_eiger . The linked dataset is called data, so this is an issue with the nexus writer calling the link the wrong thing
- [ ] Eiger data should be the default dataset so it is automatically plotted during visualisation
- [ ] Plan parameters should be stored in metadata
- [ ] A sensible user should be added to metadata

No files or tests are named. Start by locating the NeXus writer and the Eiger data-writing path, then review the unchecked requirements for the default dataset and metadata; done means those remaining checklist items are implemented and the recorded Eiger data uses the expected path and values.

Written by the indexing model from the issue text.