Fix erroneous ufloat warning during fitting

[Tom-Willemsen]

Description of work

Fix a warning of the form:

Using UFloat objects with std_dev==0 may give unexpected results.

during fitting. This is caused by lmfit/lmfit-py#999 .

Ticket

n/a

Labels

Add appropriate label(s) to this PR

• 'bluesky-Semver-Major' - Breaking Change
• 'bluesky-Semver-Minor' - New feature
• 'bluesky-bug' - Bug fix
• 'bluesky-documentation' - Document Changes
• 'bluesky-ignore-for-release' - Do not show in the release notes

Labels can be found of the right-hand sidebar of this PR once created

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
• Warning no longer appears

Documentation

n/a

[rerpha]

we should take this out once fixed upstream