Changed BundleSettings a priori sigma defaults from -1 to Isis::Null for...

Changed BundleSettings a priori sigma defaults from -1 to Isis::Null for consistency with the rest of the Bundle defaults for these values. Commented out xml read/write tests in BundleSolutionInfo until we determine the output file type for serialization.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@6343 41f8697f-d340-4b68-9986-7bafba869bb8