Unify flowrate / velocity interface

[jbreue16]

Our interface and internal computation of the entangled parameters velocity, cross_section_area and the flow rates specified in the connections matrix are ambiguous, the interface should be unique and explicit, any reparameterizations etc. belong to the frontend.
This PR

• removes velocity from the interface, which is then solely computed from the cross section area and flow rate. Note that this is how its done in CADET-Process and CASEMA (cross section or radius needed for error estimates in 2D models) anyways.
• introduces an optional FLOW_DIRECTION field, only -1 and 1 are accepted for forward and backward flow respectively, this also substitutes the field VELOCITY_COEFF for radial and frustum flow
• updates documentation accordingly
• before merge: update cadet-verification for this interface change and run the verify_cadet-core workflow