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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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