added nipp_pack / nipp_unpack
Temporary support for multiple formats. More is clearly needed.
WARNING: Changed meaning of length! It is now expected to be the full length of the data
plus the secondary header. It used to be the length of the data, assuming a two byte secondary header.

The NIPP library is secondary-header-agnostic. If you want to validate or add a checksum, you need to
do that yourself. It does, at the moment, assume a secondary header, it just doesn't assume it will have
a checksum in it.
Merge branch 'master' of tess.mit.edu:nicer/nipp