Allow birad singlet to be defined and run as unrestricted

[alongd]

Added number_of_radical as an attribute of Species (and Job) so birad singlets can be defined/identified. Changed the condition for determining whether to run a job as restricted or unrestricted accordingly.

[codecov]

Codecov Report

Merging #114 into master will increase coverage by 0.01%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   40.13%   40.14%   +0.01%     
==========================================
  Files          22       22              
  Lines        5511     5517       +6     
  Branches     1451     1453       +2     
==========================================
+ Hits         2212     2215       +3     
  Misses       2944     2944              
- Partials      355      358       +3

Impacted Files	Coverage Δ
arc/scheduler.py	19.57% <ø> (ø)	⬆️
arc/job/job.py	18.4% <14.28%> (+0.09%)	⬆️
arc/species/species.py	58.68% <50%> (-0.04%)	⬇️
arc/reaction.py	41.47% <0%> (ø)	⬆️

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