multiple netpbm fixes

This pull request fixes a few bugs on this format.

* previously mentioned bug on openmicroscopy/bioformats#1263 about pgm files being incorrectly marked as non-interleaved;
* crash when whitespace other than newline (and equivalents) was used to separate the header values (the specs define whitespace for delimiter  as space, CR, LF, TAB, VT, or FF)
* crash when it failed find values due to comments at end of line (even right after a value without whitespace in between)

Just as an example, the following is a valid (albeit random) PGM image:

    P2
    12# feep.ppm
     10  # feep.ppm
    15# feep.ppm


     0  0  0    0  0  0    0  0  0   15  0 15
     0  0  0    0 15  7    0  0  0    0  0  0
     0  0  0    0  0  0    0 15  7    0  0  0
    15  0 15    0  0  0    0  0  0    0  0  0
     0  0  0    0  0  0    0  0  0   15  0 15
     0  0  0    0 15  7    0  0  0    0  0  0
     0  0  0    0  0  0    0 15  7    0  0  0
    15  0 15    0  0  0    0  0  0    0  0  0
     0  0  0    0  0  0    0  0  0   15  0 15
     0  0  0    0 15  7    0  0  0    0  0  0

Lastly, the file is named PGMReader but it's actually trying to handle ppm, pgm, and pbm. Since the collective of this 3 formats is [PNM](http://netpbm.sourceforge.net/doc/pnm.html), I'd suggest renaming it to PNMReader.