Add missing check for 'degree' in BidirectionalGraphConcept. Fix the …

[anadon]

…concept checking class in the documentation for BidirectionalGraphConcept. Implementation of missing 'degree' function for filtered_graph.

I'm going through other forks and came across this.

[jzmaddock]

This looks like a low-risk merge to me - anyone else care to comment?

[jeremy-murphy]

The implementation of the new functions looks correct... but there are no additional unit tests.