[evlatests] L350 reference phase monitor

[Keith Morris]

A second L350 (evla-mib-0-df) was installed in the Master Rack.  Its 
512MHz output is connected to the L353 (ea17-L353, evla-mib-1-15) phase 
detector input.  There, it is phase-compared to the Master L350, the one 
that feeds the antennas.

This monitor point archived once per second, and is available from the
EVLA archive as

Antenna name: 		ea17-L353
Device name: 		L353
Monitor point name: 	PHASE_DET_MON

Data after 2006-Dec-27 22:00:00 (UTC) is valid.

The monitored quantity is phase detector voltage, which can be converted
to degrees as

Phase (deg) = arccos(((det_volt/10) - 0.01)/0.158)*(180/pi)

These values (DC offset 0.01V, detector constant 0.158V) are specific to
L353 SN009.