Add alias for rels and relations

[janikapeters]

Functions with both names existed with some taking the same arguments and computing the same thing.

[lgoettgens]

@fingolfin @fieker any objections?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.93%. Comparing base (4b127f8) to head (01d2cae).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2168   +/-   ##
=======================================
  Coverage   87.93%   87.93%           
=======================================
  Files         127      127           
  Lines       31776    31776           
=======================================
  Hits        27941    27941           
  Misses       3835     3835           

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[lgoettgens]

This is unfortunately incompatible with the current Oscar master as this PR here turns https://github.com/oscar-system/Oscar.jl/blob/73e4186aa042c4f719dc06752f82bdd2da1bc572/src/Modules/UngradedModules/SubquoModule.jl#L2175 into a self-call (resulting in a StackOverflow when called).

Since I still want to get this in eventually, I mark this as technically breaking, so we remember to add it to the next breaking AA release.

[fingolfin]

I am fine with this but as you say, there is a problem with doing that in OSCAR. So first we need to change OSCAR to delegate the other way there (resp. turn rels there into an alias for relations).

So someone should open an issue for OSCAR, or provide a PR, to do that.