Description of eta_circuit and the factor of pi/4

[MatthijsRoos]

The function spectrometer_sensitivity() requires the input eta_circuit, which has a default value of 0.32. In simulator.py, it is described as follows:

The loss at chip temperature, *in the circuit.*. Units: None.

This, however, may confuse the user, as eta_circuit is more formally defined as the peak transmission times a correction factor of pi/4, due to the filter being shaped as a Lorentzian. This is in line with how eta_circuit is defined in filter.py:

Average transmission over the FWHM. Equal to pi/4 times the peak transmission

To clarify its definition, I would like to suggest to add the description used in filter.py to the one in simulator.py, as users might use the latter as the reference