[N/A] Update sipnet.output precision and doc

[Alomir]

Summary

• What: Quick tweak to increase output precision to minimum 4 digits, and update output doc

How was this change tested?

Smoke and unit tests; testOutputHeader updated.

Checklist

• PR title has the issue number in it ("[#] <concise description of proposed change>")
• Tests added/updated for new features (if applicable)
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)

[infotroph]

Here's the one I'll nitpick: I doubt we need more decimal places for soil C. Given it's always greater than zero (usually thousands of g/m2, sometimes hundreds, hardly ever tens) and highly variable, differences smaller than a gram aren't usually meaningful.

Suggested change

	fprintf(out, "%8.4f ", envi.soilC);
	fprintf(out, "%8.2f ", envi.soilC);

[infotroph]

It's possible the same argument applies to other C pools, but I'm less sure which ones. Keeping higher precision for trackers/fluxes than for pools does make sense to me, to avoid truncation at short timesteps.