Add exchange rxns

[JonathanRob]

Main improvements in this PR:

[Addresses Issue #117]

Exchange reactions are added for four metabolites that are present in the extracellular compartment, but do not currently have exchange reactions transporting them to/from the boundary. These changes are implemented with the curateExchangeRxns2.m script.

The four metabolites for which exchange reactions are added:

1. 20-hydroxy-arachidonate
2. chenodiol
3. LacCer pool
4. Chylomicron Lipoprotein

In addition, the script addresses the issue of five reactions that involve transport between the boundary compartment and a non-extracellular compartment. The reaction IDs are:

1. HMR_9736
2. xenobiotics
3. arachidonates
4. steroids
5. others

Information on the new reactions added to the model is contained in the newExchangeRxns.tsv file.

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Selected devel as a target branch

[JonathanRob]

This PR also demonstrates the use of the new docModelChanges and writeModelChanges functions introduced in PR #118.

[haowang-bioinfo]

@JonathanRob this PR looks good.

Would it be better to name the files according to the targeted issue? For example, change filename from newExchangeRxns.tsv to newExchangeRxns_issue117.tsv, and curateExchangeRxns2.m to curateExchangeRxns_issue117.

[JonathanRob]

@Hao-Chalmers I think that's a good idea. It would make it quite clear which files are associated with which issues/changes.

[haowang-bioinfo]

This PR actually adds 8 exchange rxns that are claimed for addressing #117, where there are only 4 exchange rxns were proposed. It would thus be necessary to also mention the other changes (mets and exchange/transport rxns) in #117.

[JonathanRob]

@Hao-Chalmers Yes, this PR adds more exchange reactions than the 4 discussed in #117, but these are added for a different reason (as explained in the description of this PR).

The 4 exchange reactions discussed in #117 are to allow exchange of metabolites that are in the extracellular compartment, but do not currently have exchange reactions to/from the boundary. The other exchange reactions added in this PR are for a separate problem, where metabolites are being transported to/from the boundary from a non-extracellular compartment. Since this was not the topic of #117, I don't think there's a need to discuss these other reaction changes there.