Allow generate_reactions.py script to accept number of processors

[kspieks]

Motivation or Problem

This pull request resolves #1779

Description of Changes

'maxproc': args.maxproc, was added as an entry to the kwargs dictionary in generate_reactions.py

Testing

"python ~/RMG-Py/scripts/generateReactions.py -n 1 input.py" will print the expected "For reaction generation, 1 process is used" from react.py
"python ~/RMG-Py/scripts/generateReactions.py -n 2 input.py" will now print the expected "For reaction generation, 2 processes are used" from react.py
Activity monitor confirms that 2 processes are being used and indeed the run time is faster

Reviewer Tips

All feedback and suggestions are welcome :)

[codecov]

Codecov Report

Merging #1780 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1780   +/-   ##
======================================
  Coverage    32.6%   32.6%           
======================================
  Files          87      87           
  Lines       26116   26116           
  Branches     6875    6875           
======================================
  Hits         8516    8516           
  Misses      16630   16630           
  Partials      970     970

Impacted Files	Coverage Δ
rmgpy/tools/generate_reactions.py	30% <ø> (ø)	⬆️

---

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 ceadaf8...775aeda. Read the comment docs.

[ajocher]

Thanks for adding this!