AQT: Improve documentation, Add Z operations and rename AQTRemoteSimulator to AQTSamplerLocalSimulator

[pschindler]

The API for the AQT devices has been changed and should now be stable. The code for accessing the API has been changed and handling of connection errors has been improved.

This PR adds Z operations for the AQT device.

This PR addresses the issues #2194 #2091

The documentation has been imroved, possible addressing #2503

[vtomole]

#2579 can be handled in a different PR.

[Strilanc]

Overall changes look good, with little formatting inconsistencies here and there.

[pschindler]

There seems to be an issue with the Travis test. The pytest-changed-files produces following error:

========================= no tests ran in 0.00 seconds =========================
ERROR: file not found: cirq/aqt/aqt_sampler_test.py
cirq/aqt/aqt_simulator_test.py
The command "check/pytest-changed-files" exited with 4.

https://travis-ci.com/quantumlib/Cirq/jobs/258921745

[pschindler]

I see the same issue on my local machine.
There seems to be an issue with check/pytest-changed-files
If I remove the " from the last line, it seems to work:

pytest ${rest} ${changed[@]}

Should I open an issue for this?

[vtomole]

@pschindler Merged in #2584 and into your branch. That check passes here now.

[vtomole]

LGTM

Requested changes have been addressed.