Issue 93: Model amount of children

[ManuelAlvarezC]

Resolve #93

Allow to model the amount of child rows for each parent row and using it when sampling.

[codecov-io]

Codecov Report

Merging #98 into master will decrease coverage by 1.46%.
The diff coverage is 72.89%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage    81.4%   79.93%   -1.47%     
==========================================
  Files           5        5              
  Lines         613      608       -5     
==========================================
- Hits          499      486      -13     
- Misses        114      122       +8

Impacted Files	Coverage Δ
sdv/modeler.py	91.06% <100%> (-2.01%)	⬇️
sdv/sdv.py	53.33% <60%> (+0.7%)	⬆️
sdv/sampler.py	71.37% <69.51%> (-2%)	⬇️

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