Drop support for multiple plane parameters with ForecastSolar for simplicity

Summary

Currently the user can pass multiple plane parameters to ForecastSolarNode instantiation.

This however might be inconsistent with the idea of nodes as single datapoints. Furthermore the internal implementation by lists, has introduced complexities and might continue to do so.

Dropping support altogether might therefore ensure low error-proneness and consistent use of nodes as unitary datapoints. Correct data aggregation should be implemented in read_series methods.