Emulator for xarcos

Emulates a spectrometer with two ALMA boards of 8 single spectrometers
each. It waits for connections on the 1234 port, and uses the cpld2
protocol as described in the XARCOS software document. The personality
download command (not used except during initialization in the embedded
computer) is not supported.

Input section (ADC) is not emulated at all (at least for now). 
Each ADC provides thus exactly the same signal.
The signal is a roughly realistic (polynomial) passband, 
that is the same for the four channels (XX, YY, re(Xy), im(XY)),
with only a different scale factor.
Scale factor with respect to the XX channel are:
1.0, 0.8, 0.25, -0.1 respectively. 

A gaussian line, with nominal frequency of 18.75 MHz, 
FWHM $\approx0.12$ MHz and amplitude $\approx19$\% of the 
continuum is also added. 

The overall amplitude, band, center frequency depends on 
the dbbc gain setting. The dbbc band is not considered 
(for now) in the passband shape computation. 

Integration time is realisticly emulated, i.e. the end 
integration flags for both the FFT spectrometer and the 
TP detector comes true after the correct time. 
Overflow flag, random pattern generator, and ADC total power
detectors are not emulated at all. 
