adding a new density scatter plot and putting limits on how much data glance will be willing to plot in traditional scatter plots and hex plots

moving the reports and some other things I missed the first time around to use the new constants module

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@203 8a9318a1-56ba-4d59-b755-99d26321be01

moving a huge pile of strings into constants modules and refactoring several groups of functions out of compare.py into different modules organized by their logical function

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@201 8a9318a1-56ba-4d59-b755-99d26321be01