adding extra checks when variable data is loaded to prevent crashing and some cosmetic changes to html report output

added global and variable attributes to report pages; fixed bug in the io module that was causing global attributes to never be loaded; corrected the behavior for purple marking circles behind small numbers of mismatch points and added that behavior to some other kinds of mismatch plots

"glance info --parsable" now outputs the results in tab deliminated format, repeating the filename on each line, making it easier to parse programmatically. Additional fields may be added later!

At the moment only supports "info".  May add support for other formats later.

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