Data concentration plot

Hi Aaron,

If you are interested, I added a new subtool that returns the type of plot one sees in MinKNOW during the sequencing run with the sum of data over each bin. 

poretools/poretool_main.py was edited to include the subcommand "data_conc" and poretools/dataconc.py was added to the repertoire. 

dataconc.py uses the matplotlib and pandas libraries. I kept with the structure and lingo you used in hist.py. data_conc can actually write to any image extension type that matplotlib allows, but I arbitrarily restricted it to pdf and jpg to avoid errors thrown from erroneous extensions.

best,

  John