allow specifying initial uncertainties

[bgctw]

and cover case of empty vector of correlations.

[codecov]

Codecov Report

Attention: Patch coverage is 65.51724% with 30 lines in your changes missing coverage. Please review.

Project coverage is 76.28%. Comparing base (750ab58) to head (81c7f44).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cholesky.jl	55.55%	24 Missing ⚠️
src/HybridProblem.jl	37.50%	5 Missing ⚠️
src/init_hybrid_params.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   75.59%   76.28%   +0.68%     
==========================================
  Files          20       20              
  Lines         545      565      +20     
==========================================
+ Hits          412      431      +19     
- Misses        133      134       +1     

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