related Service and Process

This PR treats Work and Service in equal way. We can see both individual and group agents as vf:Process (or distinguish their default process if needed #16). We often call input Work and output Service but to have seamless IPO chains we should treat this type of resource equally on input and output.

I can happily change give/receive terminology to something else, just something to start with and reflect in modeling Work/Service as resource https://github.com/valueflows/resource/issues/15

I also changed direction of arrows in VOWL, it should show direction of predicate (directed labelled edge in a graph) and not our interpretation of represented value flow.