readpvpheader.m enhancements

[peteschultz]

This pull request adds some features to the mlab/util/readpvpheader.m:

• The first argument can be either a file id or a filename. If a filename, it opens the file, reads the header, and closes the file.
• The pos argument can be a negative position, which specifies the number of bytes backward from the end of the file.
• For weight files, the header includes fields for nxp, nyp, nfp, min, max and numpatches, instead of lumping them into "additional".