Improve StatsSpec and fix an issue with macro did

[junyuan-chen]

StatsSpec now has only one type parameter.

@did no longer assumes StatsSpec and functions that prepare the arguments are in the Main module.

A new helper function _get_default is added for merging default values into the named tuple of arguments.

[codecov]

Codecov Report

Merging #10 (98f410a) into master (e653da5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files           8        8           
  Lines         506      510    +4     
=======================================
+ Hits          504      508    +4     
  Misses          2        2           

Impacted Files	Coverage Δ
src/StatsProcedures.jl	99.20% <100.00%> (+0.01%)	⬆️
src/did.jl	100.00% <100.00%> (ø)

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