Allow defining a TS species simply with .xyz guesses

[alongd]

instead of defining TSGuess objects

[codecov]

Codecov Report

Merging #124 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   40.48%   40.43%   -0.05%     
==========================================
  Files          22       22              
  Lines        5748     5755       +7     
  Branches     1527     1530       +3     
==========================================
  Hits         2327     2327              
- Misses       3041     3046       +5     
- Partials      380      382       +2

Impacted Files	Coverage Δ
arc/species/species.py	58.15% <0%> (-0.4%)	⬇️
arc/scheduler.py	20.23% <0%> (ø)	⬆️
arc/reaction.py	41.69% <0%> (ø)	⬆️

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