Improve the interface of defining new StatsStep types with groupargs

[junyuan-chen]

To specify arguments required by each StatsStep, the previous practice is to define new methods for namedargs. The returned NamedTuple is then passed to _getargs.

The improved design proposed here mainly make the following changes:

1. Rename _getargs to groupargs for more accurate description of its role.
2. Replace namedargs with three new functions that cover most scenarios: required, default and transformed.

[codecov]

Codecov Report

Merging #4 (bb86668) into master (b4efeff) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #4   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files           8        8           
  Lines         459      465    +6     
=======================================
+ Hits          457      463    +6     
  Misses          2        2           

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/StatsProcedures.jl	99.18% <100.00%> (+0.01%)	⬆️
src/procedures.jl	100.00% <100.00%> (ø)

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