Release version 2.20.1: "Stripped Bolt Head"

[rcurtin]

This automatically-generated pull request adds the commits necessary to make the 2.20.1 release.

Once the PR is merged, mlpack-bot will tag the release as HEAD~1 (so that it doesn't include the new HISTORY block) and publish it.

Or, well, hopefully that will happen someday.

When you merge this PR, be sure to merge it using a rebase.

Changelog

• Clarify return values for different callback types (#383).

• Fix return types of callbacks (#382).

• Minor cleanup for printing optimization reports via Report() (#385).

[conradsnicta]

@rcurtin Instead of a mere patch bump, suggest to change the version to 2.21.0 or even 2.30.0.

A patch bump usually indicates small bug fixes. To my eyes at least, the callback mechanism has significant changes that go beyond that.

There is also a risk that the changes can introduce regressions in user code (user defined callbacks that we don't see), so it would be better to have a clearer delineation of the "old" and "new" callback mechanism within the ensmallen version number.

[rcurtin]

Yeah, good point---let me do that instead.