Refactor prepare_switch method (backport #1417)

[mergify]

Refactored prepare_switch mechanism. This way it should be easier to understand and to extend.

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This is an automatic backport of pull request #1417 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   ur_robot_driver/include/ur_robot_driver/hardware_interface.hpp

Unmerged paths:
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	both modified:   ur_robot_driver/src/hardware_interface.cpp

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