Reimplement Solar class

[Tom-Kingstone]

Description:

in #262, the Solar class was removed as it was unstable and confusing to use.
Propose reworking and then reintroducing the class, then applying the methods currently in solar.py to the new class.

[charliebrookerBH]

Hi team, @tg359, @Tom-Kingstone, I regularly get asked about this as part of computational team planning - is this something we're still pursuing? Is it complete?
Would be great to get it complete and off of the TODO list