[RF] Faster Hesse in RooFit by advertising which params are independent

[guitargeek]

This reduces the time to run Hesse in the ATLAS Higgs benchmark from
123 s to 92 seconds.

Given that some models take hours for this, this is a significant
improvement for the user experience.

Further improvement is possible by analyzing the computation graph a bit
more to find more independent parameters (e.g., the different gammas for
stat uncertainties from different bins).

Benchmark results

The Higgs combination benchmark takes 31 s to minimize, plus 79 s for the Hessian with ROOT master. It was 130 seconds with ROOT master just a few days ago, before some optimization PRs got merged. This PR reduces the time for hesse() to 57 seconds.

[github-actions]

Test Results

    18 files      18 suites   4d 11h 12m 15s ⏱️
 2 725 tests  2 724 ✅ 0 💤 1 ❌
47 351 runs  47 350 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 3fa8122.

♻️ This comment has been updated with latest results.

[dpiparo]

The error seems an unresolved symbol on Win

2024-09-14T07:01:53.2104016Z      Creating library Z:/foobar/lib/libRooFitCore.lib and object Z:/foobar/lib/libRooFitCore.exp
2024-09-14T07:01:53.4308777Z RooXYChi2Var.obj : error LNK2001: unresolved external symbol "public: virtual void __cdecl RooAbsArg::fillVariableGroups(class RooFit::VariableGroups &)const " (?fillVariableGroups@RooAbsArg@@UEBAXAEAVVariableGroups@RooFit@@@Z) [Z:\foobar\build\roofit\roofitcore\RooFitCore.vcxproj]
2024-09-14T07:01:53.4311585Z RooAbsTestStatistic.obj : error LNK2001: unresolved external symbol "public: virtual void __cdecl RooAbsArg::fillVariableGroups(class RooFit::VariableGroups &)const " (?fillVariableGroups@RooAbsArg@@UEBAXAEAVVariableGroups@RooFit@@@Z) [Z:\foobar\build\roofit\roofitcore\RooFitCore.vcxproj]

[dpiparo]

LGTM, thanks for this improvement! Let's not forget to give to this feature the right emphasis in the RNs.