Consider adding `ensure_midnight_datetime` utility function

[andresmtkraken]

Context

A new datetime utility function (ensure_midnight_datetime) will be added to xocto repository that might be moved or replicated in the src/timezone_tools/_dates.py file.

Relevant resources

• xocto PR: Contracts: add ensure_midnight_aligned helper to localtime octoenergy/xocto#265
• Slack thread

[samueljsb]

Could you explain what problem this function solves, please? It looks a lot like an alias for the existing midnight method, but with some surprising behaviour/API with None values.