Cleaning up scalar support (Polyhedral Geometry)

[alexej-jordan]

This PR includes:

• re-factoring for support types like PointVector and Halfspace to consistently integrate their handling into polyhedral geometry and Oscar while improving performance and reducing code cuplication.
• the helper type union scalar_type_or_field which is used by any constructor that allows different scalar types
• adjustments on tests, e.g. removal of nf_elem scalars (which are not embedded)
• adjustments on several access functions of the SubObjectIterator to increase type stability

Also, while the code that has made it into the master (greatly) does its job for the majority of its destined functionality, some edge cases in the context of quadratic number fields did not work as intended. As of now, a big part of this is already is solved with this PR, although not entirely.

[codecov]

Codecov Report

Merging #2690 (ebf08ab) into master (d0e775e) will increase coverage by 7.05%.
Report is 17 commits behind head on master.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #2690      +/-   ##
==========================================
+ Coverage   73.63%   80.68%   +7.05%     
==========================================
  Files         455      455              
  Lines       64536    64465      -71     
==========================================
+ Hits        47520    52013    +4493     
+ Misses      17016    12452    -4564     

Files Changed	Coverage
src/PolyhedralGeometry/solving_integrally.jl	ø
src/PolyhedralGeometry/linear_program.jl	50.00%
src/PolyhedralGeometry/helpers.jl	68.75%
src/PolyhedralGeometry/Cone/properties.jl	72.00%
...hedralGeometry/SubdivisionOfPoints/constructors.jl	84.61%
src/PolyhedralGeometry/PolyhedralFan/properties.jl	90.00%
...c/PolyhedralGeometry/PolyhedralFan/constructors.jl	90.47%
src/PolyhedralGeometry/Cone/constructors.jl	90.90%
src/PolyhedralGeometry/Polyhedron/properties.jl	93.87%
...PolyhedralGeometry/PolyhedralComplex/properties.jl	94.73%
... and 9 more