Fixing the number of clicks when a Gaussian state is measured with threshold detectors

[nquesada]

In Appendix A.3. of https://arxiv.org/pdf/1902.00462.pdf a formula is provided for fixing the mean number of clicks of a Gaussian state when measured with threshold detectors.
It would be good to add this feature into The Walrus.

[nquesada]

We can also now do torontonian sampling specifying the mean of number of clicks. For this we have two options:

Option 1: Change the function torontonian_sample_graph(A, n_mean, samples=1, max_photons=30, pool=False) from quantum so that the argument n_mean becomes n_clicks specifying the mean number of clicks instead of the mean photon number.

Option 2: Write a new function that does torontonian sampling from a graph and takes as argument n_clicks.

I prefer option 1 but it would require some reworking in the sf apps. What do @trbromley and @josh146 think?

[nquesada]

Addressed in #136