Improved gradients

[mcocdawc]

• Fixed a bug in the calculation of gradients.
• Added docstrings.

[codecov[bot]]

Codecov Report

Merging #86 (4004bde) into master (ae281f5) will decrease coverage by 0.16%. Report is 6 commits behind head on master. The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   65.14%   64.99%   -0.16%     
==========================================
  Files          26       26              
  Lines        2743     2748       +5     
==========================================
- Hits         1787     1786       -1     
- Misses        956      962       +6     

Files Changed	Coverage Δ
...coord/internal_coordinates/_zmat_transformation.py	26.53% <11.11%> (-0.36%)	:arrow_down:
...cartesian_coordinates/_cartesian_class_get_zmat.py	87.09% <100.00%> (ø)

... and 2 files with indirect coverage changes