Changed PaletteColorMappingSaxReader so that it no longer throws an exception...

Changed PaletteColorMappingSaxReader so that it no longer throws an exception if an unrecognized palette color mapping element is encountered.   Instead a warning message is printed about the unrecognized element so that parsing can continue.  This problem was encountered when new palette color mapping elements were added but the data file was read with an older version of wb_view.