LieAlgebras: Add Demazure character

[janikapeters]

Implemented by the request of @lgoettgens and @gfourier
I am still writing documentation and tests

[lgoettgens]

Just for future reference: @janikapeters is a student assistant at RWTH Aachen under the supervision of @gfourier and me working on project A24, in particular in extending the BasisLieHighestWeight experimental project to work for Demazure modules.

[codecov]

Codecov Report

Attention: Patch coverage is 97.64706% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.32%. Comparing base (4d753d8) to head (a326c1b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
experimental/LieAlgebras/src/RootSystem.jl	96.72%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4339      +/-   ##
==========================================
+ Coverage   84.31%   84.32%   +0.01%     
==========================================
  Files         649      649              
  Lines       86345    86430      +85     
==========================================
+ Hits        72799    72882      +83     
- Misses      13546    13548       +2     

Files with missing lines	Coverage Δ
experimental/LieAlgebras/src/LieAlgebraModule.jl	88.83% <100.00%> (+0.32%)	⬆️
experimental/LieAlgebras/src/LieAlgebras.jl	100.00% <ø> (ø)
...rimental/LieAlgebras/test/LieAlgebraModule-test.jl	100.00% <100.00%> (ø)
experimental/LieAlgebras/src/RootSystem.jl	93.73% <96.72%> (+0.25%)	⬆️

[lgoettgens]

just as a small heads-up: #4344 is very probably creating some conflicts with this PR here. I can try to help you with that when you drop by my office later today

[lgoettgens]

LGTM, thanks!