Update docstrings and add type hint

[xin-huang]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 95.74468% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 88.02%. Comparing base (f506725) to head (d28cfac).

Files	Patch %	Lines
dadi_cli/Models.py	92.59%	2 Missing ⚠️
dadi_cli/BestFit.py	94.44%	1 Missing ⚠️
dadi_cli/Plot.py	66.66%	1 Missing ⚠️
dadi_cli/SimulateFs.py	93.33%	1 Missing ⚠️
dadi_cli/Stat.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   88.35%   88.02%   -0.34%     
==========================================
  Files          12       12              
  Lines        1486     1445      -41     
==========================================
- Hits         1313     1272      -41     
  Misses        173      173              

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