ENH: Raise warnings when input file contains more than 1000 materials

     To match the behavior of the original GMV program and to avoid to
     confuse those users whose files accidentally violate the GMV file
     format specification - as in those cases material values are
     subject to a (mod 1000) operation.
ENH: Reworked the tests for GMVReader plugin

     To avoid tiny picture norm deviations among platforms
     * suppressed outline representations
     * disabled orientation axes and center axes actors
     and also test reader's ability to import polygonal and tracer data.
Merge topic 'gmv_fixes' into next

ead3c7b ENH: Reworked the tests for GMVReader plugin
f83f39f ENH: Raise warnings when input file contains more than 1000 materials