Adding two leaf two stream canopy and solar rad model

[j-emberton]

Description

This PR relates to the ongoing discussion #234 and the implementation of a two-leaf canopy representation and associated solar model (Work in progress)

To do list:

• Migrate useful solar functions out of splash.solar into core.solar to extend core solar library
• Create alternative method to calculate pdf
• Implement similar radiation model as supplied R code using python libs and/or custom code
• Implement canopy model
• Integrate canopy model productivity into existing code

Fixes # N/A

Type of change

• [ x] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist

• [ ] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [ ] All tests pass: $ poetry run pytest

Further checks

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works

[davidorme]

This looks like a bit of a meta-issue (or possibly a milestone 😄 )? I think those bullet points might all stand alone as independent features.

[j-emberton]

This looks like a bit of a meta-issue (or possibly a milestone 😄 )? I think those bullet points might all stand alone as independent features.

Should I start a new issue and/or add milestones?

I'm still a bit unfamiliar with GitHub and blundering around!