Enable checkpoint restart for OutHDF5

[The9Cat]

The issue

On restart, OutHDF5 looks for enumerated snapshot files or directories independent of checkpt

The fix

Bypass enumeration check if checkpt=true and use the existing checkpoint file or directory.

[Copilot]

Pull request overview

Enables OutHDF5 to restart correctly in checkpoint mode by skipping snapshot enumeration and instead targeting the existing checkpoint file/directory.

Changes:

• Use checkpoint_<runtag> (file or directory) as the restart target when checkpt=true.
• Skip the snapshot file/directory enumeration loop when checkpt=true.
• Avoid broadcasting nbeg in checkpoint mode since snapshot indexing is not used.

Comments suppressed due to low confidence (1)

src/OutHDF5.cc:208

• The runtime_error messages thrown here say Component::initialize error, but this code is in OutHDF5::initialize(). With the new checkpoint-path logic, these errors can be triggered in checkpoint mode as well, so the message becomes more misleading. Update the prefix to OutHDF5::initialize (and consider including whether this was snapshot vs checkpoint mode).

	if (not std::filesystem::is_directory(dir_path)) {
	  throw std::runtime_error("Component::initialize error: you specified directory organization of output but the directory " + dir_path.string() + " does not exist");
	}
      }
      else {
	std::ostringstream fname;

	if (chkpt)
	  fname << outdir
		<<  "checkpoint_" << runtag << ".1";
	else
	  fname << outdir
		<<  filename << "_" << setw(5) << setfill('0') << nbeg
		<< ".1";

	std::filesystem::path file_path = fname.str();

	if (not std::filesystem::is_regular_file(file_path)) {
	  throw std::runtime_error("Component::initialize error: you specified file organization of output but the file " + fname.str() + " does not exist");
	}

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[Copilot]

@The9Cat I've opened a new pull request, #200, to work on those changes. Once the pull request is ready, I'll request review from you.