[meas] RFC comment - MD #2,3

[CresitelloDittmar, Mark]

I don't think these are very contentious, so hopefully a single response is
sufficient.

"(2) In requirement meas.003: After reading the standard, I think I
understood what that means, although I'm not sure what the reason for the
requirement is (let alone which use case it is derived from). Let me try:
"Each error instance must only be referenced by a single measurement." Is
that what you mean? If so, why?"

This means that 'Systematic' error should only show up once for any given
value.  When you allow errors from multiple sources, it opens the
possibility that any given source (systematic, statistical, etc) could show
up multiple times.  We don't want that, I think.  If there are multiple
contributors to systematic error, we can't distinguish them anyway, so
might as well consolidate them.


"(3) While the document certainly cannot be an introduction into error
calculus, I have to say I can't tell the difference between Error.statError
and Error.ranError (I've looked things up in the Wikipedia, and it says:
"Measurement errors can be divided into two components: random error and
systematic error." So... from my own experience I'd say it would be wise to
either say a few words on what's a statError and what's a ranError or, if
that's too long, perhaps point to some textbook."

I wanted to say that these come from the predecessor(s), either STC-1.33 or
Characterization's Accuracy class, but I'm not finding it.  Anyway, I know
the 3 have been in the model since at least 2016, where they show up in
discussions I had with Arnold.

Anyway.. I have no objection to reducing the set to systematic, and
statistical only.  This is consistent with what CAOM2 has, though it looks
like it has 'sys' and 'rnd'.
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