Separated out the type checking of EndToEndFlowSegments from checking
the position of references to flow sources, paths, and sinks.  Also,
completed the type checking.
Don't do type checking when linking an EndToEndFlowSegment's context.
Link any visible Context and let the validator do the type checking.
Don't do type checking when linking an EndToEndFlowSegment's
flowElement.  Link any visble EndToEndFlowElement and let the validator
do the type checking.
Don't check nested end to end flows if linking or type checking of the
ete flow segment failed.
Check the segment position before running ete flow segment validation.
Created a validation test for the type checking of an
EndToEndFlowSegment.