Changes in Chef & OBST to Accomodate Two-channel Receivers	11 Dec 97

	The following scheme has been implemented so that two channel receivers
can have separate Trec's etc without having to define two separate front end
Raw Data sections filled with the same information.  Instead of referring to
simply the frontend number, one can append a letter to distinguish between
the two channels.  1A is the first channel of Frontend #1 and 1B is the 
second channel of Frontend #1.  (If the letter is omitted, A is assumed.)

	In OBST, the RX command is used to designate which backends are 
connected to channel 1 and which to channel 2.  This is done via /CHN1 and
/CHN2 parameters.  These currently only have effect for the SIS345 receiver. 
The defaults are 37 (i.e. backends 3 & 7) for CHN1 and 45 for CHN2.  (I
would have preferred a comma separated syntax, "3,7", but I couldn't think
of a way to get SIC to parse that.)  The AOS macro will append A or B to the
frontend number when it "connects" the backends with OBSINP's CBEn command. 
This information will then be included in the Raw scan header. 

	When CHEF reads the raw header, it will make note of which backends are
connected to which frontend and which channel.  The (Cold Mode) CALIBRATE
command stores the resulting Trec separately for each backend part.  (The
Filterbanks are handeled as one backend with 3 parts.  All other backends
have but one part.)  The signal and image band opacities and atmospheric
temperatures are also stored separately so that the CLASS header will get the
actual values for each backend.  (Previously, the CLASS header got these
quantities for the last backend part processed by CALIBRATE.)

	The CHEF SUPPLY command's /FE parameter will now accept a frontend 
number plus a letter.  Thus one can SUPPLY a different GAIN_IMAGE and 
EFFICIENCY for each channel, for instance.

	The OBST CALIBATION command's /TREC parameter now has two values, one
for channel 1 and the other for channel 2.  (It must issue separate SUPPLY
TREC commands, one for each /FE, to CHEF.)

	Finally, the CHEF SUPPLY TREC command will now accept a value of SAME
for TREC.  If this is used, the TREC calculated by the previous Cold Mode 
CALIBRATE command will be used in the Trec Mode CAL.  (The resulting TREC
is saved separately for each backend part.)