feat(AlgebraicTopology): multicoequalizers and pushouts of simplicial sets

[joelriou]

Let X be a simplicial set. In this PR, we show that a bicartesian square in the lattice of subcomplexes of X gives a pushout in the category SSet, and "multicoequalizer diagrams" in this lattice also allow to express certain subcomplexes as the multicoequalizers in the category of simplicial sets.

From https://github.com/joelriou/topcat-model-category

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[github-actions]

PR summary b7543c7cba

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ BicartSq
+ BicartSq.isPushout
+ MulticoequalizerDiagram
+ instance : Unique (MultispanShape.ofLinearOrder Bool).L
+ isColimit
+ isColimit'
+ isColimitOfMulticoequalizerDiagram'
+ isPushout_of_bicartSq
++ in

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).