Fixes and advances from the OSCAR session in Durham

[HechtiDerLachs]

This is a draft PR with the fixes and advances in code from the OSCAR session in Durham.

In the beginning this will comprise eventual small fixes for bugs found during the preparation of the material. Eventually, this will then be extended by new functionality implemented by people there.

still WIP, with ongoing changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.49%. Comparing base (2a8e01f) to head (6e4cf75).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4314      +/-   ##
==========================================
- Coverage   84.53%   84.49%   -0.05%     
==========================================
  Files         641      645       +4     
  Lines       85563    85589      +26     
==========================================
- Hits        72332    72319      -13     
- Misses      13231    13270      +39     

Files with missing lines	Coverage Δ
...ebraicGeometry/Schemes/Divisors/AlgebraicCycles.jl	75.00% <100.00%> (-0.65%)	⬇️
...gebraicGeometry/Schemes/Divisors/CartierDivisor.jl	64.25% <100.00%> (-1.00%)	⬇️
.../AlgebraicGeometry/Schemes/Sheaves/IdealSheaves.jl	80.41% <100.00%> (ø)

... and 20 files with indirect coverage changes