Fix calculation of bipolar electrograms from unipolar

[p-j-smith]

Changes made:

• Store the correct indices of pairs of unipolar egms that make up each bipolar egm
• Previously, the bipolar egm was calculated correctly, the pairs of unipolar egm that correspond to each bipolar egm were incorrect

[codecov-commenter]

Codecov Report

Merging #151 (3372b1b) into dev (ecedeec) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #151   +/-   ##
=======================================
  Coverage   72.95%   72.95%           
=======================================
  Files          11       11           
  Lines         721      721           
  Branches       86       86           
=======================================
  Hits          526      526           
  Misses        188      188           
  Partials        7        7           

Impacted Files	Coverage Δ
openep/case/case_routines.py	85.90% <0.00%> (ø)

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