Avoid repeating first objective evaluation in fit!

[dmbates]

• Modify the obj function in fit! for a LinearMixedModel to return the previously calculated optsum.finitial when isone(iter += 1)

Did behavior change? Did you add new features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

[codecov]

Codecov Report

Base: 94.21% // Head: 94.17% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (f21e84b) compared to base (73ad7f1).
Patch coverage: 91.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
- Coverage   94.21%   94.17%   -0.04%     
==========================================
  Files          29       29              
  Lines        2765     2766       +1     
==========================================
  Hits         2605     2605              
- Misses        160      161       +1     

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	98.09% <91.66%> (-0.18%)	⬇️

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