Add sticking coefficients to CTI API docs

[speth]

The stick entry was missing from the CTI API docs. This adds it, and a short docstring to the stick class for both ctml_writer and cti2yaml.

[codecov]

Codecov Report

Merging #726 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   70.69%   70.68%   -0.01%     
==========================================
  Files         372      372              
  Lines       43605    43605              
==========================================
- Hits        30825    30824       -1     
- Misses      12780    12781       +1

Impacted Files	Coverage Δ
src/transport/GasTransport.cpp	90.37% <0%> (-0.21%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55fe223...11feb1c. Read the comment docs.