feat: add cellular succinate export via MCT proton symport

[edkerk]

Main improvements in this PR:

As requested in #424, adds cellular export of succinate.

Human-GEM has three Na-coupled succinate import reactions across the plasma membrane (MAR09610, MAR04939, MAR04938) but no export, so cytosolic succinate could not be secreted (relevant for hypoxia).

• Add MAR20192: succinate[c] + H+[c] <=> succinate[e] + H+[e], GPR SLC16A1 or SLC16A3 (ENSG00000155380 or ENSG00000141526), reversible, modelled on the existing lactate MCT reaction MAR05998. Reference PMID:35363247 (MCT-mediated succinate export).

Succinate secretion now works through the existing exchange reaction MAR09415. Mitochondrial succinate export is already possible via the reversible SLC25A10/SLC25A1 antiporters, so no mitochondrial reaction is added.

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 is the output of the MACAW test:

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.

This and a more detailed output from MACAW are also committed to data/testResults/.

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