Improve code coverage

[ramiromagno]

          We need to add test cases for these functions. We can add it in a new branch.

Code coverage

sdtm.oak Coverage: 67.52%
R/assign.R: 0.00%
R/clear_cache.R: 0.00%
R/hardcode.R: 0.00%
R/oak_id_vars.R: 0.00%
R/zzz.R: 0.00%
R/dtc_problems.R: 55.26%
R/dtc_utils.R: 60.53%
R/ct.R: 79.25%
R/dtc_create_iso8601.R: 89.00%
R/parse_dttm_fmt.R: 91.87%
R/derive_study_day.R: 91.89%
R/dtc_parse_dttm.R: 100.00%
R/recode.R: 100.00%
R/str_split.R: 100.00%

Originally posted by @rammprasad in #41 (comment)

[ramiromagno]

I will wait for the following PRs to be merged, and then branch off of main for this issue:

• Implementation of hardcode_no_ct(), hardcode_ct(), assign_no_ct() and assign_ct() #41
• assign_datetime algorithm #47