DM-33325: Update measurement uncertainties in meas_extensions_trailedSources

[langfzac]

First-pass at adding uncertainties for updated NaivePlugin measurements (length, angle, flux).

• Move flux computation to C++ layer, and adds gradient computation with respect to the other VeresModel parameters.
• Add SAFE_CENTROID flag. (Want the SdssShape centroid, so throw flag if we fall back to safe.)
• Move 'non-weighted' length computation to function and compute gradients of both length computations.
• Propagate measurement uncertainties from centroid and second-moments.
• Add tests for gradient computations
• Add monte-carlo tests for length and angle uncertainties. For now, just make sure we're close, or at least over estimate.

Note: Flux uncertainty underestimates the monte-carlo derived values (often ~50%). This will be addressed in a future PR.