Allow user to input paths to previous jobs

[amarkpayne]

fixes #347

There are many times when a user might want to calculate a species at a higher sp level using a previously calculated geometry (for example, when updating the reference database). When this is the case, it does not make sense for ARC to recalculate the geometry. ARC will submit the new sp job, but cannot submit the Arkane job because it does not know the paths to the old geometry/frequency job.

The additions in this PR allow the user to pass a dictionary with species labels as keys, and geo and freq paths in a dictionary as values. ARC then adds these species paths if given to the output dictionary before processing the species.

[amarkpayne]

@alongd let me know what you think of the implementation. Also, let me know where this should be added to the documentation.

I have tested this out in my own work, and I think it is working as desired

[codecov[bot]]

Codecov Report

Merging #393 into master will decrease coverage by 0.05%. The diff coverage is 7.69%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
- Coverage   54.39%   54.33%   -0.06%     
==========================================
  Files          34       34              
  Lines       11901    11914      +13     
  Branches     3628     3633       +5     
==========================================
+ Hits         6473     6474       +1     
- Misses       4476     4487      +11     
- Partials      952      953       +1     

Impacted Files	Coverage Δ
arc/processor.py	54.35% <0.00%> (-3.57%)	:arrow_down:
arc/main.py	63.14% <100.00%> (+0.04%)	:arrow_up:

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[amarkpayne]

@alongd hold off on merging this, there is one additional change that I would like to make