feat: add (S)-dihydroorotate transport and exchange reactions

[edkerk]

Main improvements in this PR:

Resolves #357. (S)-dihydroorotate (MAM00180) existed only in the cytosol, with no transport or exchange reaction, so the model could not represent its secretion by cells under hypoxia (PMID:30643150).

• Add extracellular metabolite MAM00180e.
• Add MAR20193: (S)-dihydroorotate[c] <=> (S)-dihydroorotate[e], reversible, no GPR (no established plasma-membrane transporter for dihydroorotate).
• Add MAR20194: exchange of (S)-dihydroorotate[e].

Both reactions are reversible, enabling import and export. The transport reaction is mass and charge balanced; the exchange is open by construction. No genes are affected.

The metabolite's ChEBI (CHEBI:17025, the acid form) reflects the model-wide ion/acid convention and is left unchanged.

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Selected develop as a target branch
• Any removed reactions and metabolites have been moved to the corresponding deprecated identifier lists

[github-actions]

This PR has been automatically tested with GH Actions. Here are the results of the quality-control tests:

MACAW: dead-end and duplicate tests

Starting dead-end test...
 - Found 1384 dead-end metabolites.
 - Found 1141 reactions incapable of sustaining steady-state fluxes in either direction due to these dead-ends.
 - Found 1369 reversible reactions that can only carry steady-state fluxes in a single direction due to dead-ends.
Starting duplicate test...
 - Skipping redox duplicates because no redox_pairs and/or proton_ids were provided.
 - Found 377 reactions that were some type of duplicate:
   - 0 were completely identical to at least one other reaction.
   - 13 involve the same metabolites but go in the opposite direction or have the opposite reversibility as at least one other reaction.
   - 377 involve the same metabolites but with different coefficients as at least one other reaction.

Mass and charge balance

Unbalanced reactions (excluding boundary and biomass): 277 (87 mass, 240 charge)

More detailed output is committed to data/testResults/ (macaw_results.csv and balance_results.csv).

Note: In the case of multiple test runs, this post will be edited.