Fix #299: Rounding issue for DiD results.

[anevolbap]

Closes #299 .

• Rounding round_num was applied to a string. The method itself handles the rounding.
• Tests are already done here.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 77.10%. Comparing base (2223fa0) to head (42018ca).

Files	Patch %	Lines
causalpy/pymc_experiments.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #302      +/-   ##
==========================================
+ Coverage   77.06%   77.10%   +0.03%     
==========================================
  Files          21       21              
  Lines        1378     1380       +2     
==========================================
+ Hits         1062     1064       +2     
  Misses        316      316              

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