i15-1: Call real centring analysis

Following https://github.com/DiamondLightSource/crystallography-bluesky/pull/76 we are now calling analysis at the end of doing a centring scan. However, we're calling a dummy analysis, we should instead call the centring analysis and use the result.

To make this work nicely it would be good if @RJCD-Diamond could provide us with the centre of the scan, as read from the nexus file

## Acceptance Criteria
- The centring analysis is called on the centre sample plan
- We use the result as returned from analysis

Review pull request 76 and trace the centre sample plan through the centring scan to find where the dummy analysis is called. Check how the centring analysis result is represented, then verify that the real analysis is called and its returned result is used; the Nexus-file centre is relevant context.

Written by the indexing model from the issue text.