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

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@227 8a9318a1-56ba-4d59-b755-99d26321be01

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@220 8a9318a1-56ba-4d59-b755-99d26321be01

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@214 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