Fix bug still present in CutSample.m

#733 was faulty because it did not account for changes in the first line when more than 1 MH file was present. Now, FirstDraw really denotes the first draw (as opposed to the last draw as before). The subsequent computation of FirstMhFile is then also correct.
Merge pull request #756 from JohannesPfeifer/CutSample

Fix bug still present in CutSample.m