Add 'raw' output option to codadump

[svniemeijer]

This would allow dumping of a binary subblock of data to stdout or a file.
This would only work for data where we currently track the byte offset/lengths of the data inside the original file. So at least for pure ascii/binary data files.

But we should also try to see if we can't use this to e.g. dump specific submessages of grib files or dump specific subsections of xml files.