Updated spins.py to include model fitting with optional terminator-defined ellipse shadowing correction (#190)

* Updated spins.py to include model fitting with optional terminator-defined ellipse shadowing correction on the ssHG1G2 model

* Added minor changes to the func_ssHG1G2_terminator and illum_and_shadowed_areas documentation and ensured ruff doesn't complain

* Fix function description

* Add notes

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Co-authored-by: JulienPeloton <[email protected]>

Author: OdysseasXenos