[wpimath] ChassisSpeeds Discretize() and FromRelative() instance methods missing in C++

[KangarooKoala]

#7115 added instance method versions of discretize() and fromRelative() that modify the instance in-place instead of creating a new instance, and the old static versions were deprecated. This was only done in Java, though, and should be done in C++ too for parity.

Good first issue for those who either know C++ or are willing to learn.

[calcmogul]

It was mainly done to reduce Java GC pressure, which C++ doesn't have.

[KangarooKoala]

It would still be nice to maintain API parity, though.

[calcmogul]

I disagree. Mutable APIs on aggregates generally confuses users (see: the Java units API and our recent backslide to an immutable API).