Consider the project directory when reading ARCSpecies or TSGuess xyz

[alongd]

Allow users to simply write

reactions:
  - label: <some label...>
    xyz:
    - TS1.gjf
    - TS2.gjf

and populate the reaction user guesses correctly from the given files considering the project directory as needed. Tests added.

[codecov[bot]]

Codecov Report

Merging #683 (3bbb786) into main (af4a9d6) will increase coverage by 0.20%. The diff coverage is 97.77%.

@@            Coverage Diff             @@
##             main     #683      +/-   ##
==========================================
+ Coverage   73.47%   73.68%   +0.20%     
==========================================
  Files          99       99              
  Lines       26492    26524      +32     
  Branches     5542     5547       +5     
==========================================
+ Hits        19466    19543      +77     
+ Misses       5663     5631      -32     
+ Partials     1363     1350      -13     

Flag	Coverage Δ
unittests	73.68% <97.77%> (+0.20%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
arc/scheduler.py	21.31% <ø> (+1.06%)	:arrow_up:
arc/species/conformers.py	74.36% <ø> (ø)
arc/reaction_test.py	99.58% <96.42%> (-0.13%)	:arrow_down:
arc/reaction.py	66.79% <100.00%> (+0.55%)	:arrow_up:
arc/species/converter.py	76.25% <100.00%> (+0.05%)	:arrow_up:
arc/species/species.py	71.98% <100.00%> (+0.65%)	:arrow_up:

... and 3 files with indirect coverage changes

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