Reduction mod p for schemes

[HechtiDerLachs]

@simonbrandhorst : Have a look at the test file, please. Do you think this goes in the right direction? If yes, I will extend to CoveredSchemes.

[simonbrandhorst]

I think this goes in the right direction.

[codecov]

Codecov Report

Merging #2387 (26e292a) into master (b37fbb8) will increase coverage by 0.11%.
The diff coverage is 97.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2387      +/-   ##
==========================================
+ Coverage   71.44%   71.55%   +0.11%     
==========================================
  Files         384      385       +1     
  Lines       52652    52798     +146     
==========================================
+ Hits        37615    37780     +165     
+ Misses      15037    15018      -19     

Impacted Files	Coverage Δ
src/AlgebraicGeometry/Schemes/Methods.jl	0.00% <0.00%> (ø)
...ry/Schemes/ProjectiveSchemes/Objects/Attributes.jl	86.86% <ø> (ø)
...eometry/Schemes/AffineSchemes/Morphisms/Methods.jl	81.81% <100.00%> (+4.39%)	⬆️
...ometry/Schemes/AffineSchemes/Objects/Attributes.jl	86.89% <100.00%> (+0.18%)	⬆️
...cGeometry/Schemes/AffineSchemes/Objects/Methods.jl	94.30% <100.00%> (+2.44%)	⬆️
...ometry/Schemes/CoveredSchemes/Morphisms/Methods.jl	83.60% <100.00%> (+39.42%)	⬆️
...Geometry/Schemes/CoveredSchemes/Objects/Methods.jl	70.83% <100.00%> (+7.67%)	⬆️
...raicGeometry/Schemes/Covering/Morphisms/Methods.jl	100.00% <100.00%> (ø)
...ebraicGeometry/Schemes/Covering/Objects/Methods.jl	81.66% <100.00%> (+2.16%)	⬆️
src/AlgebraicGeometry/Schemes/Glueing/Methods.jl	94.04% <100.00%> (+1.86%)	⬆️
... and 3 more

... and 7 files with indirect coverage changes

[HechtiDerLachs]

This should be ready to go as a first version from my side. Let's see if the tests go through. Then I'll take care of switching off some internal tests and eventually rename everything to base_change. But you can already go through the code to complain about other stuff.

[simonbrandhorst]

Suggested change

	struct BaseChangeGlueingData{T1, T2}
	struct BaseChangeGlueingData{T1, T2} # used for LazyGlueing

[simonbrandhorst]

Could you please also add a test when one works over a non-trivial base and restricts to a fiber, i.e.
some base like $K[x,y]$ and then some base change to $K[x]$ via $y \mapsto 15, x \mapsto x$ or similar.

[HechtiDerLachs]

Could you please also add a test when one works over a non-trivial base and restricts to a fiber, i.e.
some base like

I probably could. However, our schemes are not really equipped with a backend to handle these kind of relative schemes. I don't think that translating something like K[x,y] to a coefficient ring in Singular works. And if it does, it's probably not effective.

Edit: I could try to do something with my RingFlattenings, though... Just kidding. I suggest to leave this as is.