Fix error in `get_backbone_vector`

[chrisjonesBSU]

I made a last second change that returned the absolute value of the vector, but that broke the expected behavior in flowermd. I don't think it is really necessary to return the absolute value.

[codecov[bot]]

Codecov Report

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

Comparison is base (9cff1f1) 96.01% compared to head (3613e48) 96.01%. Report is 1 commits behind head on master.

Additional details and impacted files

[](https://app.codecov.io/gh/cmelab/cmeutils/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) ```diff @@ Coverage Diff @@ ## master #73 +/- ## ======================================= Coverage 96.01% 96.01% ======================================= Files 8 8 Lines 677 677 ======================================= Hits 650 650 Misses 27 27 ``` | [Files](https://app.codecov.io/gh/cmelab/cmeutils/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) | Coverage Δ | | |---|---|---| | [cmeutils/geometry.py](https://app.codecov.io/gh/cmelab/cmeutils/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab#diff-Y21ldXRpbHMvZ2VvbWV0cnkucHk=) | `96.92% <100.00%> (ø)` | |