Implementation of pbraw export

Implementation of export of data to pbraw format, using the structure of the SLAC Archiver Appliance.
Please consider redoing these three functions get_*() using dictionaries.  Which can be created easily with something like:

> _lut_key1, _lut_key2 = {}, {}
> for T in ALL_TYPE_DESCRIPTIONS:
>    _lut_key1[T.KEY1] = T
>    _lut_key1[T.KEY2] = T
> del T