Simple addition to controll multi-axis odrives such as 3.6

[Mathew3000]

Hi, I am currently using the UART implementation of the library with an ODrive 3.6 and needed the possibility to change the controlled axis.

To change as little as possible I added a command to set and get the kMotorNumber variable.

Set and Get state also now check this variable.

This can for sure be done in a more elegant way but I needed a quick solution.

[samuelsadok]

Note that ODrive v3.6 is a legacy product, which is why it's not actively supported in this library. Therefore I am also hesitant with a change like this. It needs to not affect maintainability / support load and not look like ODrive v3.6 is officially supported (because we don't have the resources for that). I need to think about those points. Meanwhile some comments.

Btw, if we would officially support a multi-axis ODrive with this library, it's likely that we'd want a slightly different usage approach: I'm thinking of one high-level device object that provides device-level functionality and one or more child "axis" objects that provide axis level functionality.

[samuelsadok]

this would have to be documented, especially that it can be ignored for all current-generation ODrives

[Mathew3000]

See above, could be mitigated with the overload

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[samuelsadok]

Based on internal discussions, the previously stated concerns remain:

• we envision a different approach to multi-axis support in the future
• ODrive v3.6 is not officially supported by this package, so even though it might work right now, we don't want to create the false impression that it is supported, because it will break when we make future changes.

Therefore I think users of ODrive 3.6 are best served if someone makes and maintains an independent unofficial fork of this package. For these reasons I'm closing this PR but appreciate the proposed contribution.