Add direct sums of Lie algebras

[lgoettgens]

This currently contains #3894 and needs to get rebased once that is merged. So please skip the first commit when reviewing this.

This is only a first sketch, there will possibly be changes (and documentation) at a later point.
I am aware that this does not (yet) is completely as #3059 wants it to be, put this is in the big part due to some generic functionality not being there to get used and me thus needing to include some of the generic functions myself (but restricted to the types here). All of this can be significantly simplified once there has been some more progress towards #3059.

[codecov]

Codecov Report

Attention: Patch coverage is 75.60976% with 20 lines in your changes missing coverage. Please review.

Project coverage is 83.87%. Comparing base (f2697ea) to head (9f45893).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3896      +/-   ##
==========================================
- Coverage   83.88%   83.87%   -0.02%     
==========================================
  Files         588      590       +2     
  Lines       80922    81000      +78     
==========================================
+ Hits        67885    67942      +57     
- Misses      13037    13058      +21     

Files	Coverage Δ
experimental/LieAlgebras/src/LieAlgebraHom.jl	85.10% <100.00%> (+0.16%)	⬆️
experimental/LieAlgebras/src/LieAlgebras.jl	100.00% <ø> (ø)
...ental/LieAlgebras/test/DirectSumLieAlgebra-test.jl	100.00% <100.00%> (ø)
...xperimental/LieAlgebras/src/DirectSumLieAlgebra.jl	72.60% <72.60%> (ø)

... and 1 file with indirect coverage changes

[HereAround]

Thank you @lgoettgens looks great to me.