Some modifications to BoltHumanoid be on par with Solo12

[luator]

Description

• Add some missing methods: is_ready(), set_max_current(), send_target_joint_{position,velocity,gains}
• Make dynamic_graph_manager really optional
• Make slider box optional. NOTE: this changes the default behaviour to not use the slider box. Explicitly pass a serial port or "auto" to initialize() if you want to use it!

How I Tested

Used in conjunction with https://github.com/open-dynamic-robot-initiative/robot_interfaces_bolt

I fulfilled the following requirements

• [ ] 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]

@jviereck @MaximilienNaveau @Lhumd @majidkhadiv @thomasfla Is one of you still working with this repository or knows someone who does? It would be great if someone could review my changes :).

[MaximilienNaveau]

@luator has it been tested on the real robot? with/without slider?

[luator]

@MaximilienNaveau Good point, I think I did not test the "slider box disabled" case. I'll do that when I'm in the lab again.

[luator]

Now also tested with slider box disabled, everything works as expected.