Improve logging integration to NiryoRobot

[ctmbl]

This PR proposes several minor improvements to the use of the logging facility by the NiryoRobot class.

Mainly it allows the user to pass a custom logger, allowing to:

• use different loggers for different NiryoRobot instances (main feature)
• get rid of the default handler that prints to stdout without any formatting

It also suggest to lower the logging level of the No new conveyor detected message from ERROR to WARNING in accordance with the logging documentation definition of these levels.

[jmottier-niryo]

Those are nice additions to PyNiryo's logging