Fix RDataReporting::observables_likelihood

[dweindl]

Fixes #2627. ReturnData.sllh was always NaN because it wasn't 0-initialized.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.03%. Comparing base (1716181) to head (326cbac).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2633   +/-   ##
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  Coverage    77.03%   77.03%           
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  Files          332      332           
  Lines        22970    22971    +1     
  Branches      1478     1478           
========================================
+ Hits         17695    17696    +1     
  Misses        5264     5264           
  Partials        11       11           

Flag	Coverage Δ
cpp	74.15% <100.00%> (+<0.01%)	⬆️
cpp_python	33.47% <100.00%> (+<0.01%)	⬆️
petab	38.31% <100.00%> (+<0.01%)	⬆️
python	72.11% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/rdata.cpp	95.61% <100.00%> (+<0.01%)	⬆️