Fix writing and loading openCARP datasets

[p-j-smith]

Changes made:

• Fix adding egms to openCARP dataset - case.electric.internal_names is now the same as case.electric.names, rather than an array of empty strings
• Fix creation of empty Field data - default size is now 0 rather than None. Using None creates a 0d array with a single value, rather than a 1d array with no values.

[codecov-commenter]

Codecov Report

Merging #166 (e04227a) into dev (65985a6) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #166   +/-   ##
=======================================
  Coverage   67.29%   67.29%           
=======================================
  Files          11       11           
  Lines         795      795           
  Branches      110      110           
=======================================
  Hits          535      535           
  Misses        239      239           
  Partials       21       21           

Impacted Files	Coverage Δ
openep/data_structures/case.py	52.38% <0.00%> (ø)
openep/data_structures/electric.py	71.73% <ø> (ø)
openep/io/readers.py	64.28% <0.00%> (ø)

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