Fixed socetlinescankeywords to generate more ephemeris points and quaternions...

Fixed socetlinescankeywords to generate more ephemeris points and quaternions if needed; increase precision of output SOCET Set keyword values; redefine pixel location to use for the image center ephemeris time; and temporarily compute spacecraft velocities.  Fixes 1672

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5272 41f8697f-d340-4b68-9986-7bafba869bb8