Add low bound to the sum of flows for transport flows

[gnawin]

Model

• Enforce free signs only for transport flows, the rest flows will have a low bound of 0.
• Add a lower bound of 0 for the sum of outgoing flows, if one of them is a transport flow, for producers, conversions, and storages. Exclude assets with UC because the bound is already applied by constraint min_output_flow_with_unit_commitment.
• Add a lower bound of 0 for the sum of incoming flows, if one of them is a transport flow, for conversions, and storages. Since UC does not have bounds for incoming flows, so assets with UC are not excluded.

Documentation

• Update concepts.
• Update mathematical formulation.

Related issues

Closes #1159, #1168

Checklist

• I am following the contributing guidelines

• Tests are passing

• Lint workflow is passing

• Docs were updated and workflow is passing

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.66%. Comparing base (4d26355) to head (fa3ad37).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/variables/flows.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1172      +/-   ##
==========================================
- Coverage   97.78%   97.66%   -0.13%     
==========================================
  Files          30       30              
  Lines        1131     1158      +27     
==========================================
+ Hits         1106     1131      +25     
- Misses         25       27       +2     

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[gnawin]

For some reason, codecov complains. However, when I run generate_coverage() locally, there is no difference between main and this branch - all % match. Codecov says there is less coverage for ramping-and-unit-commitment.jl, but it is 100%.

Here is the report for main:

Here is the report for this branch:

[datejada]

@gnawin Thanks for the changes, they look good. Minor comments on the text and two main comments:

1. One for now: I see that you are not testing the new constraints, aren't you? I mean, you don't change any data in the case studies that are actually generating these constraints. I am saying this because I was expecting changes in the MPS files of one of the case studies (Norse, perhaps?). Any particular reason that I might be missing? otherwise I would suggest testing the new constraints by modifying the Norse.
2. long-term, so not in this PR (for future record): some constraints might be just bounds, if the asset only has one input or output. So, if we identify those cases beforehand, we can avoid creating constraints that are actually bounds. As I said, for the future 😃

[gnawin]

Closed in favor of #1178