repr: use indexes as basis for printing out coords rather than values

[FabianHofmann]

No description provided.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (513bddb) 87.29% compared to head (59ac041) 87.30%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   87.29%   87.30%           
=======================================
  Files          14       14           
  Lines        3118     3119    +1     
  Branches      706      707    +1     
=======================================
+ Hits         2722     2723    +1     
  Misses        289      289           
  Partials      107      107           

Files Changed	Coverage Δ
linopy/constraints.py	88.45% <ø> (ø)
linopy/expressions.py	89.01% <ø> (ø)
linopy/variables.py	90.73% <ø> (ø)

... and 2 files with indirect coverage changes

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