Define a destructive measurement channel

[Strilanc]

Related to https://github.com/quantumlib/Cirq/issues/882 but I see that issue as an optimization of the simulator as opposed to defining a channel.

[dabacon]

Is this for measurement and reset? Or for actual destructive measurement?

Channel for measurement and reset to 0 has A_0 = [[1, 0], [0, 0]] A_1 = [[0, 1], [0, 0]] (you can also reset to different basis, so it might be useful to build the correct primitive version of these that are basis dependent).

If it is a real destructive measurement (qubit disappears, like a photon) then probably then the channel goes to a one dimensional space A_0 = [[1, 0]] A_1 = [[0, 1]]

[dabacon]

Related #1357

[tanujkhattar]

We support measurement and reset channel now via ResetChannel.

Is the destructive measurement channel useful in practice and something that should be added? If not, we can close this issue.