Fix histogram normalization

[chrisjonesBSU]

This PR fixes how we normalize the histogram heights returned by numpy.histogram.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3953734) 96.29% compared to head (3b93b43) 96.28%. Report is 2 commits behind head on master.

Additional details and impacted files

[](https://app.codecov.io/gh/cmelab/cmeutils/pull/78?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) ```diff @@ Coverage Diff @@ ## master #78 +/- ## ========================================== - Coverage 96.29% 96.28% -0.01% ========================================== Files 8 8 Lines 783 781 -2 ========================================== - Hits 754 752 -2 Misses 29 29 ``` | [Files](https://app.codecov.io/gh/cmelab/cmeutils/pull/78?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) | Coverage Δ | | |---|---|---| | [cmeutils/plotting.py](https://app.codecov.io/gh/cmelab/cmeutils/pull/78?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab#diff-Y21ldXRpbHMvcGxvdHRpbmcucHk=) | `100.00% <100.00%> (ø)` | |