verifyPresentation method takes a challenge as an argument but then doesn't use it

[jchartrand]

The verifyPresentation method takes a 'challenge' as an argument, but then doesn't use the challenge. This is because verifyPresentation always invokes the underlying VC library with an AssertionProofPurpose (which doesn't use a challenge) and the VC library uses that AssertionProofPurpose in preference to the supplied challenge (i.e, ignores the challenge). So the challenge goes unused.

The verifyPresentation method should check for the challenge, and if present, not use the AssertionProofPupose.

This issue was identified by @fabrii in #9