implemented structural variant fields

[ejacox]

Implements ga4gh/ga4gh-schemas#772 (and ga4gh/ga4gh-schemas#827)

[codecov-io]

Codecov Report

Merging #1579 into master will increase coverage by <.01%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #1579      +/-   ##
==========================================
+ Coverage   85.95%   85.95%   +<.01%     
==========================================
  Files          34       34              
  Lines        7510     7519       +9     
  Branches      943      947       +4     
==========================================
+ Hits         6455     6463       +8     
- Misses        857      858       +1     
  Partials      198      198

Impacted Files	Coverage Δ
ga4gh/server/datamodel/variants.py	94.37% <84.61%> (-0.07%)	❌

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