[RF] Make the code emitted by codegen locale-independent

[guitargeek]

The codegen backend formats the numbers it writes into the generated C++ with default-constructed streams, which follow the global locale. Under a comma-decimal locale (a German one, say) the emitted literals came out as "0,5": that does not compile, and inside a call argument list the comma even turns one argument into two, so the generated code silently changes meaning instead of failing.

Imbue the classic locale on every stream whose output ends up in the generated code: CodegenImpl's doubleToString(), which formats the value of RooConstVar/RooRealVar and the analytical integrals, plus CodegenContext's buildArg() for double arguments, buildArgSpanImpl() for the emitted arrays (where a grouping separator would corrupt integers the same way), and the stream that renders the node descriptions into the code comments.

Add a regression test that generates the code for a RooGaussian with constant parameters and for a RooChebychev (whose observable range is passed to the generated call as a plain double) under a comma-decimal locale, built from a custom numpunct facet because no such OS locale is installed everywhere. Both must produce exactly the code they produce under the classic locale. This is the first test for the codegen package, so it also adds roofit/codegen/test/.

🤖 Done with the help of AI

[github-actions]

Test Results

    21 files      21 suites   3d 12h 45m 18s ⏱️
 3 861 tests  3 860 ✅ 0 💤 1 ❌
72 051 runs  72 049 ✅ 1 💤 1 ❌

For more details on these failures, see this check.

Results for commit 77488f1.