Expose System::getNumOfObsForOutput / getObsForOutput for in-process embedders

[wshlavacek]

Summary

Two one-line accessors on NFcore::System for index-based read access to
the output observable list:

int          System::getNumOfObsForOutput() const;
Observable * System::getObsForOutput(int index) const;

Both are inline header-only and read directly from the existing
obsToOutput vector.

Why

The current public surface for reading observables from a System* is
getObservableByName(std::string), which assumes the caller already
knows the observable names. The CLI output path
(outputAllObservableNames() plus .gdat headers) covers end-users
running the NFsim binary, but doesn't help an in-process consumer
that wants to capture observable counts directly from a System.

These accessors close that gap without changing simulation behavior or
adding any new state.

What it changes

+ int getNumOfObsForOutput() const { return static_cast<int>(obsToOutput.size()); }
+ Observable * getObsForOutput(int index) const { return obsToOutput.at(index); }

• Pure reads of existing private state via std::vector::size() and
  std::vector::at().
• No new fields, no new dependencies, no header pollution.
• The standalone CLI is unchanged: outputAllObservableNames and the
  .gdat output path are not touched, and existing validate/ tests
  continue to pass.

Example

NFcore::System *sys = NFinput::initializeFromXML(xml_path, /*...*/);
sys->prepareForSimulation();
sys->stepTo(10.0);

for (int i = 0; i < sys->getNumOfObsForOutput(); ++i) {
    NFcore::Observable *obs = sys->getObsForOutput(i);
    std::cout << obs->getName() << " = " << obs->getCount() << "\n";
}

Verified

• macOS clang build clean.
• validate/basicModels/v39.xml smoke run produces unchanged output.