We need to be able to do two things:

1. Given an identifier (root name, observation number, whatever), read
   in the whole FPA (e.g., for phase 1 and 3).  This means that we
   need to be able to identify the files for each chip, and the
   extensions.

2. Given a file, identify what it is: observation number + chip/cell.
   This is so that we can identify the filter, airmass, etc.  We can't
   expect that these will always be specified in the chip level FITS
   headers, but might be stored in a database under the observation
   number.

That is, we need to be able to identify the components given the
whole, and we need to identify the whole given the component.



Here is the list of IPP concepts, each of which must be specified somewhere.

FPA-level:
FPA.AIRMASS		Airmass at which the observation is made (boresight)
FPA.FILTER		Filter used
FPA.POSANGLE		Position angle
FPA.RA			Right Ascension of boresight
FPA.DEC			Declination of boresight
FPA.RADECSYS		System of RA,Dec (e.g., J2000 or ICRS)
FPA.NAME		An identifier (e.g., observation number) for the FPA instance

Chip-level:
CHIP.NAME		The name of the chip (unique within the FPA) --- set at read
CHIP.FPA		FPA identifier

Cell/Readout-level:
CELL.NAME		The name of the cell (unique within the parent chip) --- set at read
CELL.CHIP		Chip identifier --- might be set at read
CELL.DATE		Date of observation start
CELL.TIME		Time of observation start
CELL.EXPOSURE		Exposure time of image
CELL.DARKTIME		Dark time for image
CELL.BIASSEC		Overscan region(s)
CELL.TRIMSEC		Trim region
CELL.GAIN		CCD gain (e/ADU)
CELL.READNOISE		CCD read noise (e)

There is no reason why concepts higher up can't be specified lower
down (multiple times, at the risk of introducing the potential for
value to become out of sync).  And there is no reason why (most)
concepts lower down can't be specified higher up, and simply be
inherited.  The exception to this is the OBSNUM (observation number),
which allows an FPA to be identified given a cell, etc.


These are optional:

DB.HOST		Database host name (for psDBInit)
DB.NAME		Database name (for psDBInit)
DB.USER		Database user name (for psDBInit)
DB.PASSWORD	Database password (for psDBInit)

