Introduce DummyRing, DummyCommutativeRing, and DummyField

[zickgraf]

The interface might still change and I would like to add dummy homalg rings at some point, but for now this is useful for having a dummy input for RingAsCategory.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (798a29b) 80.89% compared to head (543c349) 80.91%.

Files	Patch %	Lines
CAP/gap/DummyImplementations.gi	95.28%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1508      +/-   ##
==========================================
+ Coverage   80.89%   80.91%   +0.02%     
==========================================
  Files         487      487              
  Lines       62307    62381      +74     
==========================================
+ Hits        50404    50478      +74     
  Misses      11903    11903              

Flag	Coverage Δ
ActionsForCAP	64.09% <ø> (ø)
AttributeCategoryForCAP	88.88% <ø> (ø)
CAP	84.53% <95.93%> (+0.06%)	⬆️
CartesianCategories	93.33% <ø> (ø)
CompilerForCAP	94.40% <ø> (ø)
ComplexesAndFilteredObjectsForCAP	73.60% <ø> (ø)
FreydCategoriesForCAP	81.23% <ø> (ø)
GeneralizedMorphismsForCAP	61.60% <ø> (ø)
GradedModulePresentationsForCAP	44.58% <ø> (ø)
GroupRepresentationsForCAP	72.05% <ø> (ø)
HomologicalAlgebraForCAP	73.21% <ø> (ø)
InternalExteriorAlgebraForCAP	93.08% <ø> (ø)
LinearAlgebraForCAP	66.69% <ø> (ø)
ModulePresentationsForCAP	68.46% <ø> (ø)
ModulesOverLocalRingsForCAP	90.70% <ø> (ø)
MonoidalCategories	90.53% <ø> (ø)
ToricSheaves	21.79% <ø> (ø)

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