[RF] assorted improvements

[egpbos]

This Pull request:

Changes or fixes:

Each of the four commits contains an independent improvement that I made while working on something else (error handling). They are not necessary, but I think they are useful nonetheless, so here they are.

1. MultiProcess::Messenger worker to master messages implemented slightly more efficiently.
2. Implemented full TestStatistics::RooAbsL::constOptimizeTestStatistic; these features are used rarely in Higgs fits, I think, but they are nonetheless features in non-MP RooFit fits that I now added for feature parity.
3. Extend RooAbsL test suite with comparisons to the non-MP RooFit fits (RooNLLVar) that they were intended to reimplement.
4. Extend NaNPacker test suite with some simple arithmetic demonstrations / sanity checks.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    10 files      10 suites   2d 1h 27m 30s ⏱️
 2 635 tests  2 634 ✅ 0 💤 1 ❌
24 868 runs  24 867 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit e00aded.

[egpbos]

CI failure (roottest-python-basic-datatype) is unrelated to this PR.

[guitargeek]

Thank you very much for the improvements, and also for the code formatting!