[Bug] Sign convention for Control.load()

[laurensvalk]

Describe the bug
Control.load() currently returns a measure of "load" by just returning the (slow-moving average of) the PID value.

But the "load" is actually the opposite direction, so it should be returning -pid_avg.

We could either leave it and document it, or fix it and update that it changed since V3.2.

[laurensvalk]

Here's what we can do to avoid breaking changes:

• Deprecate Control.load(), Control.stalled(), and Control.done(). I've been meaning to do this since all of these are now available directly on the motor object anyway.
• Deprecate = remove from docs, but leave them in the implementation as-is for this release.

[laurensvalk]

This has been fixed as described above, more details in changelog.