Replace operation_weights in CategoryConstructor by a better mechanism

[zickgraf]

When using the string "default" for create_func_*, one must now set the option underlying_category, from which the weight is determined. If create_func_* is a function, one can (optionally) set a weight by returning a pair of a string and an integer (instead of only a string).

@mohamed-barakat This PR partially implements a suggestion you had: You suggested to pass an underlying category to CategoryConstructor from which the weights can be taken. I argued that this does not make sense in general. However, while working with the weights, I noticed that it indeed does make sense in a specific case, namely when using the string "default" for create_func_*. This PR now requires to set the option underlying_category when using the string "default". This sets the correct weights, which probably requires changes like recompiling code. Please tell me when I can merge, i.e. when you have to time to adjust things in CategoricalTowers.

[mohamed-barakat]

Das gefällt mir sehr gut, vielen Dank. Ich bräuchte allerdings etwas Zeit um die notwendigen Anpassungen zu machen.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.20%. Comparing base (e6b89d6) to head (90aa08b). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
CAP/gap/CategoryConstructor.gi	80.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1678      +/-   ##
==========================================
- Coverage   83.81%   81.20%   -2.62%     
==========================================
  Files         545      559      +14     
  Lines       72747    79552    +6805     
==========================================
+ Hits        60975    64597    +3622     
- Misses      11772    14955    +3183     

Flag	Coverage Δ
ActionsForCAP	64.04% <ø> (ø)
AttributeCategoryForCAP	90.45% <ø> (ø)
CAP	86.28% <92.30%> (-0.04%)	:arrow_down:
CartesianCategories	89.43% <ø> (+0.06%)	:arrow_up:
CompilerForCAP	95.90% <100.00%> (ø)
ComplexesAndFilteredObjectsForCAP	73.55% <ø> (ø)
FreydCategoriesForCAP	82.20% <ø> (ø)
GeneralizedMorphismsForCAP	61.76% <ø> (ø)
GradedModulePresentationsForCAP	44.62% <ø> (ø)
GroupRepresentationsForCAP	71.92% <ø> (ø)
HomologicalAlgebraForCAP	73.21% <ø> (ø)
InternalExteriorAlgebraForCAP	93.09% <ø> (ø)
LinearAlgebraForCAP	52.92% <ø> (?)
ModulePresentationsForCAP	69.88% <ø> (ø)
ModulesOverLocalRingsForCAP	90.70% <ø> (ø)
MonoidalCategories	89.84% <ø> (+0.14%)	:arrow_up:
ToricSheaves	21.79% <ø> (ø)

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[zickgraf]

@mohamed-barakat You can merge this PR once the changes required in CategoricalTowers are implemented.