Use Datetime

[rafaqz]

This PR allows using DateTime directly. Its a little bit awkward still, and maintains the original interface. May need a bit more thought but we should proably merge it and come back to it as that becomes clearer - rather than make it perfect straight away.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.27%. Comparing base (423aa8e) to head (7b39d70).

Files with missing lines	Patch %	Lines
src/solar_radiation.jl	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   93.43%   93.27%   -0.17%     
==========================================
  Files           5        5              
  Lines         701      714      +13     
==========================================
+ Hits          655      666      +11     
- Misses         46       48       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.