Feature: Complementary channel

[vprusso]

Given a map (quantum channel) compute the complementary map.

As an example, this functionality is present in the QETLAB package. While the implementation there is in MATLAB, the spirit of the implementation should be quite similar to the implementation for toqito:
http://www.qetlab.com/ComplementaryMap

Write the functionality for this task in toqito/channel_ops/complimentary_channel.py and ensure proper unit tests for this feature are written in tests/test_channel_ops/test_complimentary_channel.py. Also ensure that the automated docs are updated by placing the line toqito.channel_ops.complimentary_channel in docs/channels.rst under Operations on Quantum Channels.