implement value2d histo with one example

Hi Burt and Ted,

  This step is based on histos.value, and works for 2d histograms. It works fine for me (phi vs. eta example included), and I thought it might be generic enough to be in supy.steps.histos.
Please let me know if it sounds reasonable. We might want to add some checks on the size of the input variables?
Cheers,

Davide