CalibratedAxis: improve averageScale javadoc
CalibratedAxis: rename equation methods

The "equation" method was renamed to "generalEquation" and
"calibratedEquation" was renamed to "particularEquation" to match
usual mathematical terminology.
Rename UncalibratedAxis to IdentityAxis

Rather than thinking of this implementation as "uncalibrated" (which
would simply be a TypedAxis that does not implement CalibratedAxis at
all), let's repurpose this class as the identity function (y = x) since
that's already what it was in practice. The only difference conceptually
is that it is fine for an IdentityAxis to have a non-null unit; this
actually makes the implementation more concise, so no issue there.