pointreg.xml: Added ClearCache calls to free up the large amount of cube data...

pointreg.xml: Added ClearCache calls to free up the large amount of cube data in memory and increased the number of simultaneous open cubes to 500. This program should have a much smaller memory footprint. 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2106 41f8697f-d340-4b68-9986-7bafba869bb8