Changed the method to read the header of the PGM file - this turned out to  cause problems with some platforms. No idea why, but the new method does  not rely on a temporary file.
Changed the method to read the header of the PGM file - this turned out to 
cause problems with some platforms. No idea why, but the new method does 
not rely on a temporary file.

svn path=/trunk/; revision=10845

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot