Opendss unittests

[kavuribhavya]

These are the initial unit tests, they are not fully completed but having them here for reference.

[codecov-io]

Codecov Report

Merging #215 into master will increase coverage by 2.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   43.92%   46.24%   +2.31%     
==========================================
  Files          75       75              
  Lines       17036    19009    +1973     
==========================================
+ Hits         7483     8790    +1307     
- Misses       9553    10219     +666

Impacted Files	Coverage Δ
ditto/readers/json/read.py	86.53% <0%> (-2.01%)	⬇️
ditto/readers/synergi/read.py	3.16% <0%> (-1.58%)	⬇️
ditto/writers/ephasor/write.py	89.39% <0%> (-1.03%)	⬇️
ditto/readers/opendss/read.py	62.07% <0%> (+1.06%)	⬆️
ditto/writers/json/write.py	95.07% <0%> (+1.88%)	⬆️
ditto/readers/cyme/read.py	63.38% <0%> (+2.19%)	⬆️
ditto/readers/abstract_reader.py	59.75% <0%> (+3.65%)	⬆️
ditto/writers/opendss/write.py	66.62% <0%> (+4.32%)	⬆️
ditto/readers/gridlabd/read.py	61.43% <0%> (+5.88%)	⬆️

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[NicolasGensollen]

We need to solve #210 #211 #213 and #214 which were raised by this PR.