[Python][UHI] add flow option to .values()

[siliataider]

.values() gets access to the bin contents, now you can optionally pass flow=True to get the flow bins.
.values() returns a view on the histogram's data, except for the special cases of TH*C and TProfile* for which we get a copy.

[github-actions]

Test Results

    20 files      20 suites   3d 9h 55m 14s ⏱️
 3 777 tests  3 773 ✅ 0 💤  4 ❌
73 809 runs  73 741 ✅ 0 💤 68 ❌

For more details on these failures, see this check.

Results for commit 84f9b72.