Move the calculateAttenuation method from MCInteractionVolume onto the
Track class so it's available for situations where you're not doing a
Monte Carlo simulation eg non-simulated absorption weighted path length
New X-ray absorption correction algorithm may also benefit from this

Also return tracks from MCInteractionVolume::calculateBeforeAfterTrack in
different way so it's easier to mock the Track class in the MCAbsorptionStrategy
unit tests
Also had to use testing::invoke to return the mocked track class because
SetArgPointee didn't seem to work in a polymorphic way