add test and fix to calc_theta_delta_bounds for multiconductor case

[ccoffrin]

Closes #870

[codecov]

Codecov Report

Merging #871 (257b5eb) into master (e9712cd) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #871      +/-   ##
==========================================
+ Coverage   93.79%   93.81%   +0.02%     
==========================================
  Files          43       43              
  Lines        9893     9893              
==========================================
+ Hits         9279     9281       +2     
+ Misses        614      612       -2     

Files Changed	Coverage Δ
src/core/data.jl	91.85% <100.00%> (+0.12%)	⬆️

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