Fix visit merging

[svittoz]

Description

eds_scikit.period.stays.merge_visits function outputs non-deterministic results due to koalas sort_values(...).first() not working as expected.

We add the sort_values_first_koalas function for replacement.

Checklist

• [ ] If this PR is a bug fix, the bug is documented in the test suite.
• [ ] Changes were documented in the changelog (pending section).
• [ ] If necessary, changes were made to the documentation (eg new pipeline).