Class to backup MultiNest output

If you are running a long MultiNest job that takes a few hours or even days, MultiNest or your computer or likelihood functions can crash. 

Sometimes this isn't a problem, as MultiNest scans can be resumed from the closest checkpoint.

However, that checkpoint could be hours ago, or much much worse, the crash might happen while MultiNest is writing the data files, in which case resuming is impossible as the numbers of points in the [root]ev. dat and [root].points won't agree with the [root]resume.dat.

I've written a class, based on your tracker classes, to backup the MultiNest output periodically. It also checks that it got a good backup, i.e. that the lengths of the data files agreed with the [root]resume.dat.