Non-numeric characters will be skipped over instead of causing ascii2isis to...

Non-numeric characters will be skipped over instead of causing ascii2isis to hang. When the input file is not large enough to fill the specified size of the output cube, ascii2isis will display an error message. Fixes #2066.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6272 41f8697f-d340-4b68-9986-7bafba869bb8