Combine 2q parallel XEB into one methods to simplify the XEB workflow

[NoureldinYosri]

This way the notebook https://quantumai.google/cirq/noise/qcvv/parallel_xeb becomes

>>> res = cirq.experiments.parallel_two_qubit_xeb(sampler)
>>> res.plot_heatmap()

[NoureldinYosri]

@eliottrosenberg this is now ready for review

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f2c6f3c) 97.81% compared to head (a91e175) 97.81%. Report is 3 commits behind head on main.

:exclamation: Current head a91e175 differs from pull request most recent head 64effd8. Consider uploading reports for the commit 64effd8 to get more accurate results

Files	Patch %	Lines
cirq-core/cirq/experiments/two_qubit_xeb.py	98.85%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6443 +/- ## ======================================== Coverage 97.81% 97.81% ======================================== Files 1111 1113 +2 Lines 97198 97326 +128 ======================================== + Hits 95078 95204 +126 - Misses 2120 2122 +2 ```

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[NoureldinYosri]

@eliottrosenberg please try again. I ran it several times and didn't run into this issue , though it can happen because of the random nature of operations. I assumed that qubit pairs are always ordered which turns out not to be true. I added a line to ensure results are accessed correctly.

[NoureldinYosri]

@eliottrosenberg I added the histogram method and removed the print statements.

[eliottrosenberg]

@eliottrosenberg I added the histogram method and removed the print statements.

Thanks, Nour!