Develop pyrealm.core.solar function radiation calc library

[j-emberton]

Proposal to breakout individual solar radiation calc functions from pyrealm.splash.solar and make these available in pyrealm.core.solar

Alternatives
Alternative would be to try and find an existing Python library to provide similar functionality and then write custom functions for any missing features such as calculation of ppfd and calculation of radiation for sub-daily intervals..
Seeing as we already have much of the code we need in Pyrealm, we might as well reuse it.

Additional context
This is in support of implementing the two-leaf canopy model

[davidorme]

In particular, the existing code in pyrealm handles array inputs, where other libraries tend to be more focussed on single locations.