Add sdv.timeseries model family

[csala]

This PR introduces a new sdv.timeseries subpackage that includes a BaseTimeseriesModel class to be used for future timeseries models and a first PAR model that implements a Probabilistic AutoRegressive model.

As a side effect, it also adds support for composite primary keys in tabular datasets.

Resolve #142
Resolve #204
Resolve #207
Resolve #208

[codecov-io]

Codecov Report

Merging #206 into master will decrease coverage by 7.26%.
The diff coverage is 12.38%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   83.00%   75.74%   -7.27%     
==========================================
  Files          24       27       +3     
  Lines        1648     1847     +199     
==========================================
+ Hits         1368     1399      +31     
- Misses        280      448     +168     

Impacted Files	Coverage Δ
sdv/timeseries/__init__.py	0.00% <0.00%> (ø)
sdv/timeseries/base.py	0.00% <0.00%> (ø)
sdv/timeseries/deepecho.py	0.00% <0.00%> (ø)
sdv/demo.py	71.23% <33.33%> (-1.63%)	⬇️
sdv/metadata/table.py	85.82% <62.50%> (-3.41%)	⬇️
sdv/constraints/base.py	91.66% <0.00%> (+1.19%)	⬆️
sdv/tabular/base.py	85.93% <0.00%> (+18.75%)	⬆️

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