Feature: Is channel extremal

[vprusso]

A function that checks whether a quantum channel possesses the property of being extremal or not. Refer to section 2.2.4 of https://cs.uwaterloo.ca/~watrous/TQI/TQI.pdf for reference.

The function would be located in channels/channel_props/is_extremal.py and would return True if the channel is extremal and False otherwise. Tests for this should go in tests/test_channel_props/test_is_extremal.py. It is expected that the input could be provided as a channel, Kraus operators, or a Choi matrix. Depending on the form of the channel provided as input, the function should processes the input accordingly.