I15-1: Add calibrations and backgrounds

Currently we have a background field in the nexus file. We will also need to take calibration, which are analysed differently. Therefore we should have the field be something like:

```
experimentType: literal ["Background", "Calibration", "Collection"]
```
rather than:
```
background: bool
```

## Acceptance Criteria
- We can make distinct calibration vs. background collections

Start by locating the nexus file handling and the existing background field, then trace how collections are represented and analysed. Replace the background-only distinction with the three experiment types described in the issue, and verify that calibration and background collections can be created and remain distinct.

Written by the indexing model from the issue text.