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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   96.01%   96.01%           
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  Files           8        8           
  Lines         677      677           
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  Hits          650      650           
  Misses         27       27           

Files	Coverage Δ
cmeutils/geometry.py	96.92% <100.00%> (ø)