This change makes the MonteCarloAbsorption algorithm ignore masked detectors
in the input workspace - in line with the behaviour of other correction algorithms
such as MayersSampleCorrection and CalculateCarpenterSampleCorrection
The algorithm outputs a masked spectra with all values equal to zero for each masked
spectrum in the input workspace. The Divide algorithm ignores masked spectra so if the
correction workspace is used as the rhs of a divide then it will be OK