adding basic data objects and making difference analysis their responsibility; minimal integration to keep them working with the rest of the code

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@110 8a9318a1-56ba-4d59-b755-99d26321be01

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@108 8a9318a1-56ba-4d59-b755-99d26321be01