factor out and adjust firstguess in lambda fit

[veni-vidi-vici-dormivi]

See #427 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.03%. Comparing base (9b0b76b) to head (5da479b).
Report is 28 commits behind head on main.

Files	Patch %	Lines
mesmer/mesmer_m/power_transformer.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
- Coverage   87.90%   82.03%   -5.88%     
==========================================
  Files          40       44       +4     
  Lines        1745     1937     +192     
==========================================
+ Hits         1534     1589      +55     
- Misses        211      348     +137     

Flag	Coverage Δ
unittests	82.03% <0.00%> (-5.88%)	⬇️

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