Better qualimap axes

Main change here is that the x axis for most qualimap plots is now determined automatically when run without an explicit parameter, instead of using a hard default. How this is decided varies on the type of data, typically `2 x median` value or something similar. This code is entirely self contained now, so much better than earlier attempts at doing this.

Second small change is the addition of `plot.close()` statements. This makes no difference when run on the command line, but when the function is imported into another script and used to generate a lot of plots it dramatically improves memory usage.
And again I only know this because I did it wrong a bunch of times. I never mean to seem all StackOverflow-nerd-contest about it. If I ever catch a Python error it's almost always because I broke something doing it that way at some point in the past.