Removed the REVERSE option and changed the CNET parameter to BASE. When...

Removed the REVERSE option and changed the CNET parameter to BASE.  When INPUTTYPE=CNETS, BASE will act exactly as CNET did.  However, when INPUTTYPE=LIST, the BASE parameter can now be provided in order to set an explicit base network, effectively adding it to the front of the input list.  If the explicit base network also appears in the input list, it will be ignored there to avoid adding it twice.  If not value is provided for BASE, the first element in the list is considered the base network; fixes #180

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4251 41f8697f-d340-4b68-9986-7bafba869bb8