Add highest_root(R::RootSystem) for convenience

[felix-roehrich]

No description provided.

[lgoettgens]

Is the highest weight a well-defined concept for redicuble root systems? If not, it would be good to restrict this to irreducible types.
And I think this should get exported.
(I'll come back to you latest after my vacation)

[lgoettgens]

I addressed my comments. Could you please have another look @felix-roehrich ?

[felix-roehrich]

Lgtm, thanks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.73%. Comparing base (c2562fc) to head (9e9eff8).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4789      +/-   ##
==========================================
+ Coverage   84.57%   84.73%   +0.16%     
==========================================
  Files         676      681       +5     
  Lines       91285    91554     +269     
==========================================
+ Hits        77201    77579     +378     
+ Misses      14084    13975     -109     

Files with missing lines	Coverage Δ
src/LieTheory/RootSystem.jl	85.50% <100.00%> (+0.74%)	⬆️

... and 25 files with indirect coverage changes

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