Implementation ticket for RFC-333.
Find and fix all incorrect uses of Sigma and Err in the stack and related documentation, to bring it all in line with the DPDD.
Fix "Sigma" and "Err" in schema(s)
Fix "Sigma" and "Err" in the stack software
Change "fluxSigma" to "fluxErr" everywhere
Note that, for earned value reasons, we can't actually do the work on this epic. I'm migrating DM-10933 to DM-14447; the other tickets will move too, when we get to them.