Zero-initialise all members of Motor

[luator]

Description

Prevents random behaviour of the motors if an application does not explicitly set all the control-related values (including kp/kd, etc.) to zero before enabling the motors.

How I Tested

I use this branch to fix an issue of motors of the Bolt robot screeching upon initialisation.

I fulfilled the following requirements

• [x] All new code is formatted according to our style guide (for C++ run clang-format, for Python, run flake8 and fix all warnings).
• [x] All new functions/classes are documented and existing documentation is updated according to changes.
• [x] No commented code from testing/debugging is kept (unless there is a good reason to keep it).

[luator]

@thomasfla I'm merging now as this issue is causing some trouble for us (and likely others as well). Let me know in case there is any problem with the changes, then I'll fix them in a follow-up PR.