Use `getindex` for vector creation

[albinahlback]

Now ZZ[1, 2, 3, 4] should create a vector. Still allows for getting generators via S[i]. Will be overloaded with specific methods since this is very generic.

Solves #2052

[thofma]

Isn't R[1] ambiguous? And what about R[:x]?

[albinahlback]

Isn't R[1] ambiguous?

Yes, but it will always default to gens(R, 1).

And what about R[:x]?

It will still work as VarName is stricter than Any.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.36%. Comparing base (2479164) to head (acc4fd3). :warning: Report is 120 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2053   +/-   ##
=======================================
  Coverage   88.36%   88.36%           
=======================================
  Files         125      125           
  Lines       31551    31552    +1     
=======================================
+ Hits        27880    27881    +1     
  Misses       3671     3671           

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[lgoettgens]

Isn't R[1] ambiguous?

Yes, but it will always default to gens(R, 1).

It is still very unintuitive for users. And as far as I can see, the syntax does not allow to create singleton vectors. (What about empty vectors?)

[fingolfin]

Thank you @albinahlback . If we started from scratch, I'd like to have this; but due to the history and existing features, unfortunately this is not something we feel we should add to Nemo.

Note that one can do ZZ.([1,2,3,4]) which is slightly less efficient and slightly longer but for interactive use hopefully "good enough".