Lie algebras: more progress

[lgoettgens]

Contains the following changes/additions:

• new: isomorphism(FPGroup, ::WeylGroup) via Coxeter relations. Already known information like is_finite and order gets set in the result as well to allow for better handling in GAP.
• new: conversions and arithmetic between RootSpaceElem and WeightLatticeElem
• new: dot(::WeightLatticeElem, ::WeightLatticeElem)
• change: dominant_character, character, and tensor_product_decomposition now use the root system, if available, and only fall back to GAP otherwise. (@benlorenz this should resolve the rest of cases of Make LieAlgebraModule tests better gc-able #3913 (comment))

This is still a draft since I would like to wait for #4036 to get merged first to avoid conflicts.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 21 lines in your changes missing coverage. Please review.

Project coverage is 84.76%. Comparing base (0a1d296) to head (a3d8640).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
experimental/LieAlgebras/src/RootSystem.jl	90.95%	18 Missing ⚠️
experimental/LieAlgebras/src/CoxeterGroup.jl	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4053      +/-   ##
==========================================
+ Coverage   84.64%   84.76%   +0.11%     
==========================================
  Files         612      614       +2     
  Lines       83250    83966     +716     
==========================================
+ Hits        70469    71170     +701     
- Misses      12781    12796      +15     

Files with missing lines	Coverage Δ
experimental/LieAlgebras/src/LieAlgebraModule.jl	91.07% <100.00%> (+0.27%)	⬆️
experimental/LieAlgebras/src/LieAlgebras.jl	100.00% <ø> (ø)
experimental/LieAlgebras/src/Types.jl	96.44% <100.00%> (ø)
experimental/LieAlgebras/src/WeylGroup.jl	92.28% <100.00%> (+3.46%)	⬆️
...rimental/LieAlgebras/test/LieAlgebraModule-test.jl	100.00% <100.00%> (ø)
experimental/LieAlgebras/test/RootSystem-test.jl	97.46% <100.00%> (+0.17%)	⬆️
experimental/LieAlgebras/test/WeylGroup-test.jl	97.77% <ø> (ø)
experimental/LieAlgebras/src/CoxeterGroup.jl	72.22% <81.25%> (+72.22%)	⬆️
experimental/LieAlgebras/src/RootSystem.jl	88.35% <90.95%> (+1.89%)	⬆️

... and 10 files with indirect coverage changes