Update twoQ_clifford_error function

[nkanazawa1989]

Summary

fix #367

Details and comments

• internally convert gate count dictionary into list of gate counts by assuming the conventional (u1, u2, u3, cx) gate set and qubit ordering.
• unittest is updated.

[nkanazawa1989]

I added both

• calculate_1q_epc (input EPG from error model -> output EPC)
• calculate_2q_epc (input EPG from error model -> output 2Q EPC out of entire sequence)

which have interfaces conform to

• calculate_1q_epg (input EPC -> output EPG)
• calculate_2q_epg (input EPC -> output 2Q EPG)

As @ShellyGarion mentioned in #367, *_epc funcs can be used for validation, while *_epg funcs can be used for evaluating calibration.

[nkanazawa1989]

added deprecation warning to the old twoQ_clifford_error, but this function is also updated to take new gate count input in dictionary format.

[chriseclectic]

LGTM. Couple of non-blocking comments if you would like to change, and a question

[chriseclectic]

It's better to break the string over multiple lines like this was previously.

[nkanazawa1989]

I noticed signature of QiskitError is *message and we can feed multiple lines, but that of warnings is message, category=None, stacklevel=1, source=None. for warnings we need to feed one long message with \.

[mtreinish]

This isn't actually the case, @chriseclectic is correct the \ here is not necessary and I'm actually surprised that one of the linters didn't complain about it. You can break the string over multiple lines as long as it's inside parenthesis. It will automatically concatenate any strings until the first , which indicates the next argument. This is pretty easy to test in the Python REPL too if you want to test it.

[nkanazawa1989]

@mtreinish Thanks for following up. I ran local tox -epy36 test and it caused an error due to this (as far as I remember that was a type error). I tried to reproduce this but the test passed after git clean -fdX. Maybe something odd was happening in my local env. If you want I'll make another PR to remove \.

[nkanazawa1989]

@chriseclectic thanks for reviewing. updated and replied to your questions.