Add support for nodes which sum to > 1

[odow]

@alikoek this is the picture we drew. (I also figured out why I couldn't find our previous discussion, I was looking for @alikok, not @alikoek.)

Preview: https://sddp.dev/previews/PR911/tutorial/capacity_expansion/#Epicycles

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (921511e) to head (c52dd2d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
- Coverage   96.57%   96.57%   -0.01%     
==========================================
  Files          29       29              
  Lines        4085     4082       -3     
==========================================
- Hits         3945     3942       -3     
  Misses        140      140              

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