derive_blfl - Address review comments

[rammprasad]

          I have fixed a few minor items. Once this PR is approved, we can address the issues below.

1. Refactor code based on suggestions - Last obs before exposure flag and baseline flag #49 (comment) and Last obs before exposure flag and baseline flag #49 (comment)
2. Validate the ISO date formats.
3. Add test case and example for baseline_visit functionality (Add test case for baseline_timepoints in derive_blfl function #95)
4. Address code issues based on suggestion - Add test case for baseline_timepoints in derive_blfl function #95 (review)

Originally posted by @rammprasad in #49 (comment)

[rammprasad]

I see this as an enhancement to the derive_blfl function; it can be addressed in the V0.2. I will not decouple this from the V0l1 release.

[ramiromagno]

Nice! Then I think we have all tasks ticked in #65.