Calculate GBT sensitivity

This will calculate the RMS thermal noise expected per spectral channel, as given by the formula equation

Receiver	Tsys	Bandwidth	Num.Channels
	K	MHz
Time on source	Time off source	Num Pols	ObsFreq.
sec	sec		GHz

(Note: set Tsys and ObsFreq to zero to use defaults for selected receiver.)
Teff = (Ton * Toff)/(Ton+Toff)

backend K1 (sampling efficiency) Select K1 K2 (weighting) Select K2

ACS 9-level 1.032 2.0 (hanning)

ACS 3-level 1.235

Spectral Processor 1.30 1.21 (uniform)

DCR 1.0

backend	K1 (sampling efficiency)	Select K1	K2 (weighting)	Select K2
ACS 9-level	1.032		2.0 (hanning)
ACS 3-level	1.235		2.0 (hanning)
Spectral Processor	1.30		1.21 (uniform)
DCR	1.0