Design solution for decibels and other "log-scale" units

[chiphogg]

This is a long-standing wish list item. We have some Aurora-internal design sketch notes which we could build on, but such as they are now, it's not really enough to begin in earnest.

The first step would be to propose the set of APIs by writing a series of usage examples. We would need to pay special attention to the distinction between "power" and "root power" units. Apparently, "decibel" encodes a different ratio depending on the dimension of unit it's applied to! Whatever APIs we design would need to satisfy this common usage while, critically, remaining "hard to use incorrectly".

We would also need to adhere to our library's core principle to avoid changing the values we store unless users explicitly request a conversion. For example: although I don't know the full set of API examples that would constitute an acceptance test, I'm pretty sure this is one of them:

EXPECT_THAT(deci(bels)(1) + bels(2), SameTypeAndValue(deci(bels)(21));

And this should be possible without ever leaving the integral domain. Explicitly, I want to rule out any solution candidates which convert the log-scale value to linear to store it.

This issue isn't likely to be the place where this design gets resolved; it's much too thorny a problem for that. We'll need to either make a Google Doc design doc, or else enable Discussions on this repo. Rather, the point of this issue is to say "yes, we're aware we don't have this; we have some ideas and plan to do it, but won't likely have time to get to it really soon".