Fix/revise the maths formulations

[Jihoon]

I find the documentation on the core math formulation is not quite standalone by itself and ends up being quite confusing to external users. It requires filling in or fixing some missing parts to better communicate the formulation doc. I open this PR, but I don't really know answers to all these.

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Some examples I found:

1. Equation COST_ACCOUNTING_NODAL

• The line explaining n^L is not relevant to the equation. Move it where it first appears, and define 'sub-node'

2. Auxiliary COMMODITY_BALANCE

• Is it missing 'variable' in the title? i.e. "Auxiliary variable COMMODITY_BALANCE"?
• Define h^A below the equation

3. I didn't check all, but multiple equations contain unexplained subscripts, which can only be guessed. (e.g. h^{OD})

4. Sometimes T^R or L^R is used to mean T^{REN} or L^{REN}.

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Also, I am not sure whether it is a standard notation in the modeling community, but denoting subsets like N(n_hat) is confusing because it usually means functions. Why don't we use N^{n_hat}, just like it does for T^{REN}.

[khaeru]

@Jihoon since you raise some uncertain issues in the PR description, but I see no response or resolution yet, I will postpone this to v3.3.

I would suggest to explicitly ask specific people for answers those questions; or guess at an appropriate answer and have a discussion with ≥1 other colleague to confirm that the changes are reasonable.

[khaeru]

Postponed to v3.4:

• Branch needs a rebase.
• Questions in the PR description need answering.

[LauWien]

I've added the WIP tag, since there is still more time/investigation needed to finish up the PR, as discussed in today's MESSAGEix meeting. Due to that and the following

• Branch needs a rebase,
• Questions in the PR description need answering,

we postpone to v3.5.

[LauWien]

today's MESSAGEix meeting

To bring the PR further, the following tasks and responsibilities have resulted:

• Writing the guidelines for code style in GAMS: @Jihoon
• Checking existing equations to be compatible: @LauWien, @behnam-zakeri

[LauWien]

As this PR is still in the "Draft" stage, we postpone to v3.6.

[khaeru]

Removing the milestone entirely. It can be re-added if/when the team decides to schedule/assign the work.

[glatterf42]

As discussed in today's meeting, we should

1. Merge this PR focussing only on the specific changes identified in its description
2. Create a tracking issue for further changes
3. Agree on a consistent notation style (possibly via a dedicated meeting; to be included in a documentation page)
4. Collect all functions and assign batches of them to multiple reviewers to split the work of ensuring the documentation is consistent with the agreed-upon style