Add example datasets nsw and mpdta

[junyuan-chen]

For generating example datasets, make.py is replaced by make.jl.

All example datasets are now stored as compressed CSV files.

[codecov]

Codecov Report

Merging #18 (1126610) into master (67f5352) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   99.69%   99.72%   +0.02%     
==========================================
  Files          10       10              
  Lines         665      736      +71     
==========================================
+ Hits          663      734      +71     
  Misses          2        2              

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/tables.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)
src/did.jl	100.00% <0.00%> (ø)
src/terms.jl	100.00% <0.00%> (ø)
src/parallels.jl	100.00% <0.00%> (ø)
src/operations.jl	100.00% <0.00%> (ø)
src/procedures.jl	100.00% <0.00%> (ø)
... and 2 more

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