Add zero-valued ref egms and ecgs when loading unipolar egms into an openCARP data set

[p-j-smith]

Changes made:

• Add ref egms and ecgs (all set to 0 everywhere) when loading unipolar egms into an openCARP data set
• This is so that the traces can be plotted in the annotation viewer of the gui

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (dev@87691b2). Click here to learn what that means. The diff coverage is 11.11%.

:exclamation: Current head 06d7dc5 differs from pull request most recent head 3da369d. Consider uploading reports for the commit 3da369d to get more accurate results

@@          Coverage Diff           @@
##             dev     #152   +/-   ##
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  Coverage       ?   72.25%           
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  Files          ?       11           
  Lines          ?      728           
  Branches       ?       88           
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  Hits           ?      526           
  Misses         ?      195           
  Partials       ?        7           

Impacted Files	Coverage Δ
openep/data_structures/case.py	60.25% <11.11%> (ø)

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