Merge adf_histogram.py from regrid_se_option branch

[brianpm]

Adds a script for generating histograms.

Uses LANDFRAC to try to do histograms for global, ocean, and land.

Saves the resulting histogram data to netCDF files (in the plot directory right now).

Plots the histograms.

Note: I haven't hooked this up to the website generation.

Note 2: @willychap requested this functionality