In slpmap, added the FILE option to the PIXRES parameter. This allows the user...

In slpmap, added the FILE option to the PIXRES parameter. This allows the user to specify that the pixel resolution from the input cube's projection should be used. Updated the documentation. Fixes #1764.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5745 41f8697f-d340-4b68-9986-7bafba869bb8