Javadoc
Function: check
Checks the multiplicity for the given arguments and returns the error
for the given connection or null if the multiplicity does not apply.
Parameters:
graph - Reference to the enclosing graph instance.
edge - Cell that represents the edge to validate.
source - Cell that represents the source terminal.
target - Cell that represents the target terminal.
sourceOut - Number of outgoing edges from the source terminal.
targetIn - Number of incoming edges for the target terminal.