Add with_std, with_counts to create_table_one

[lee-junseok]

Proposed changes

A auxiliary function create_table_one currently generates a table of mean values in a string format of "{:.2f} ({:.2f})".format(x.mean(), x.std()) and a user cannot use the mean values directly from the output table along with SMD calculated.

Issue: #747

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[jeongyoonlee]

Thanks for your contribution, @lee-junseok. Currently, the linting test is failing. Can you run the latest black and update the PR? Thanks.

[paullo0106]

@lee-junseok you can find the details in the failed log here. To fix the formatting error with black, you can check our contribution guide. Thanks

# move to the top directory of the causalml repository
$ cd causalml 
$ pip install -U black
$ black . 

[lee-junseok]

Sorry for the delay. Addressed the issue and made it ready for review. Thank you!

[jeongyoonlee]

LGTM. Thanks!