Continuous version of the Perez transposition model

[adriesse]

Is your feature request related to a problem? Please describe.
Due to the empirical look-up tables, the Perez transposition model produces small jumps in tilted irradiance under gradually evolving sky conditions, and creates a difficult challenge for reverse transposition algorithms.

Describe the solution you'd like
Create a new version of the model having smooth transitions, but without changing the overall performance w.r.t. the original.

Additional context
A publication describing a solution has been accepted for publication in the Solar Energy Journal, and this solution will be implemented here.