Reflectometry: add flood-map correction & change Guassian fit to use centre-of-mass as an initial x0 guess

[Tom-Willemsen]

Description of work

• Implement flood-map correction (to normalise for pixel efficiencies) in reflectometry detector-mapping alignment
• Improve guess for Gaussian - now uses centre-of-mass as the initial x0 estimate for the fitting routine. This makes the guess better for skewed curves, making the fit more likely to eventually converge reliably.
• Various minor tweaks as a result of "shake-down" on POLREF

Ticket

No ticket - this PR is the result of "shake-down" testing on POLREF.

Labels

• 'bluesky-Semver-Minor' - New feature

Acceptance criteria

• Pull request title is understandable for a user (e.g. scientist) reading the release notes. The PR title should be a short description of the change from a user perspective.
• Pull request has appropriate labels for automatic release-notes generation
• POLREF are happy (check with @christykinane, or believe me that I sat on POLREF until they were happy)

Documentation

See PR