For gain the negation gives the negative gain, consistent with arithmetifc
operations operating in the logarithmic domain.

For level, negation is disallowed, consistent with treating level
as a linear quantity in most contexts.

Fixes PainterQubits#429

This tweaks gain arithmetic, by making both `Gain` and `Level`
either purely linear or purely logarithmic, semantically. `Level`
was already mostly linear, with a few minor exceptions that were
removed here. `Gain` was previously logarithmic for purposes of
`+`,`-` and linear for purposes of `*`,`/` (i.e. both sets of
operations were equivalent - PainterQubits#461). This makes it purely logarithmic,
i.e. `*` and `/` between Gains are disallowed. When arithmetic
is mixed, only one of `(+,-)`, `(*,/)` is disallowed. Which
one depends on whether the non-Gain unit involved is a Level
(in which case `(+,-)` is used) or a regular Quantity (in which
case `(*,/)` is used). Note that this design leaves all examples
that are currently in the docs on this topic unchanged (though
I did rewrite the docs to clarify the new semantics). I'll take
that as a hint that this is a reasonable design that is not too
breaking to people's expectations. I also think it's a lot more
consistent.

The primary advantage of doing this is that it'll allow us to
reclaim other arithmetic operations like `*`/`/` on `Gain`,
while being quite consistent. For example, I'm workign with
an SDR that supports gains between `0.0dB` and `1.0dB` in its
amplifier, in setting increments of `1/256dB` (linearlly spaced
in dB space, i.e. logarithmically spaced). I'd like to represent
this as `0.0dB:(1/256)dB:1.0dB`, but with the current definition
of arithmetic, the definitions of range are unusable, because
the arithmetic system is iternally non-consistent.

This does not yet add those new operations, becuase some things
(like `/(::Gain, ::Gain)`) would change meaning, so I'd like to
give people a couple weeks to notice that things changed, but
I'm hoping to do that in the future.

Co-authored-by: Sebastian Stock <[email protected]>