Added Iodine as element in Chemkin input file

[sarahkha]

Motivation or Problem

Adding Iodine in the elements section of the chemkin input file

Description of Changes

file chemkin.pyx in rmgpy folder.
s = 'ELEMENTS\n'

# map of isotope elements with chemkin-compatible element representation:
elements = ('H', ('H', 2), ('H', 3), 'C', ('C', 13), 'O', ('O', 18), 'N', 'Ne', 'Ar', 'He', 'Si', 'S', 'Cl', 'I')

Testing

A clear and concise description of testing that you've done or plan to do.

Reviewer Tips

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[codecov]

Codecov Report

Merging #1825 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1825   +/-   ##
=======================================
  Coverage   43.02%   43.02%           
=======================================
  Files          80       80           
  Lines       21099    21099           
  Branches     5516     5516           
=======================================
  Hits         9077     9077           
+ Misses      11004    10990   -14     
- Partials     1018     1032   +14

Impacted Files	Coverage Δ
arkane/kinetics.py	12.14% <0%> (ø)	⬆️
rmgpy/data/statmech.py	42.2% <0%> (ø)	⬆️
rmgpy/rmg/pdep.py	12.21% <0%> (ø)	⬆️
rmgpy/data/kinetics/database.py	50.61% <0%> (ø)	⬆️
rmgpy/data/kinetics/family.py	48.35% <0%> (ø)	⬆️
rmgpy/statmech/ndTorsions.py	59.78% <0%> (ø)	⬆️
rmgpy/yml.py	15.71% <0%> (ø)	⬆️
arkane/sensitivity.py	10% <0%> (ø)	⬆️

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[amarkpayne]

This looks good, can you rebase?