Implement indexable api for topology

[juliohm]

As discussed on Zulip

[JoshuaLampert]

Looks good to me. I have only two minor suggestions.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.68%. Comparing base (079c4fc) to head (34e1b6c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/topologies.jl	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
- Coverage   87.70%   87.68%   -0.02%     
==========================================
  Files         191      191              
  Lines        5986     5994       +8     
==========================================
+ Hits         5250     5256       +6     
- Misses        736      738       +2     

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[juliohm]

I've also implemented the suggested optimizations for Domain for consistency. Waiting for tests before merging and releasing a patch.

[JoshuaLampert]

Thanks!