Reduced the maximum track-cluster distance permissable for a matched cluster.
Old value was 0.5 mm, new value is 0.1 mm.
This is motivated by the histogram of track-cluster distance.

Also added a check that the requested number of events is not greater than the total
and allow the user to input -1 from the command line to mean "all events".
Merge pull request #7 from sredford/master

Reduced the maximum track-cluster distance permissable for a matched clu...