Add dihedral distribution function

[chrisjonesBSU]

This works similarly to the bond_distribution and angle_distribution functions, but returns the dihedral distribution over a range of a trajectory.

I still need to add some unit tests.

[codecov]

Codecov Report

Merging #45 (07ebd03) into master (e774cbb) will decrease coverage by 0.15%.
The diff coverage is 97.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   99.11%   98.96%   -0.16%     
==========================================
  Files           7        7              
  Lines         340      385      +45     
==========================================
+ Hits          337      381      +44     
- Misses          3        4       +1     

Impacted Files	Coverage Δ
cmeutils/geometry.py	97.50% <93.33%> (-2.50%)	⬇️
cmeutils/structure.py	100.00% <100.00%> (ø)