val3_driver: remove magic numbers

[gavanderhoorn]

The current staubli_val3_driver VAL3 sources contain quite a few magic numbers (here, here and here for example).

Ideally, these are replaced by proper constants with semantically meaningful names, so as to avoid confusion and potential (future) errors.

Especially those related to simple_message protocol aspects are good candidates for refactoring, as they already have defined names and should be easy to replace. See Defined Constants in Message Structures of the ROS-Industrial Simple Message Protocol (candidate REP) and simple_message/simple_message.h for this.

[gavanderhoorn]

#8 will improve the situation, but there will still be a few left (such as in fetchStatus fi).