Each instrument has its own configuration file, which specifies how to
identify the instrument based on FITS headers and how to read the data
in.  The former is specified in a sub-metadata section labelled
"RULE", and is fairly straight-forward: you simply specify what FITS
keywords that instrument can be expected to have, along with the value
that it should have.  "TELESCOP" and "INSTRUME" are useful, if present,
for this purpose.

Specifying how the data is to be read in is a bit more complicated.
This is specified by several keywords.  The two most important are
"PHU" and "EXTENSIONS".  "PHU" specifies what the FITS file
represents, and may have one of two values: "FPA" indicates that
the file contains an entire focal plane array; "CHIP" indicates that
the file contains a single chip.

"EXTENSIONS" specifies what each of the extensions represents, and may
be one of the following values, with the proviso that it cannot
specify a higher level than that specified by "PHU": "CHIP" indicates
that each extension is a chip; "CELL" indicates that each extension is
a cell; "NONE" indicates that there are no extensions.

The other two important keywords are "CONTENTS" and "CELLS".  The
contents of "CONTENTS" varies according to the values of "PHU" and
"EXTENSIONS", as shown in the below table, but "CELLS" contains
metadata with values for each cell name or cell type.

PHU	EXTENSIONS	EXAMPLE		CONTENTS
===	==========	=======		========
FPA	CHIP		megacam_splice	For each chip, a list of cell types that comprise the chip.
FPA	CELL		megacam_raw	For each extension, the chip and cell type, separated by a colon.
FPA	NONE		lris_blue	For each chip, a list of cell types that comprise the chip
CHIP	CELL		gpc1_raw	For each extension, the cell type
CHIP	NONE		lris_red	The cells that comprise the chip

