spacenav_node - FULL_SCALE constant value

[ZdenekM]

File spacenav_node.cpp contains FULL_SCALE definition (512) which is used for normalizing values for "joy" topic. Non-normalized values are available on "offset" topic and it seems that my device gives 350 as maximum instead of 512. If some devices really have 512 as the max. value I would propose to make FULL_SCALE configurable using parameter.