sqzcomb 0.5.0: imperfect detection

Every spectrum the package computed so far was the noise at the extraction port; this release models what the photodiodes report. The detection module applies the standard beamsplitter model of optical loss and quantum efficiency plus additive electronic noise, both as a scalar map on quadrature variances (detected_variance, detected_squeezing_db, with dark_from_clearance_db converting a receiver's dark clearance in dB into a variance) and as the lossy Gaussian channel on xxpp covariance matrices (lossy_channel_xxpp, per-mode efficiencies allowed). required_efficiency inverts the model into the loss budget of an experiment: the minimum efficiency that still delivers a target squeezing from a given source. The tests assert the vacuum fixed point, exact composition of loss stages, physicality of the channel (symplectic eigenvalues never fall below hbar over 2), agreement of the scalar and matrix forms on a squeezed mode, and closed-form numbers. References: Leonhardt, Measuring the Quantum State of Light (Cambridge, 1997); Weedbrook et al., Rev. Mod. Phys. 84, 621 (2012).