Description

This PR addresses an issue encountered when using first_state or last_state solution objects with the output_variables option in the IDAKLU solver - for example, when supplying an starting solution to .solve() within an experiment.

When adding a first_state or last_state solution to a new solution computed with the output_variables option, a conflict arises due to the differing types of processed variables:

• first_state/last_state solutions dynamically generate ProcessedVariable objects on demand.
• New solutions (e.g. from .solve()) use ComputedProcessedVariable objects.
  These two classes currently share no functionality and cannot interoperate. Consequently, attempting to combine them to form a new solution results in failure.

This PR introduces a new abstract base class for both ProcessedVariable and ComputedProcessedVariable, providing a shared _update() method. This enables consistent combination of these objects when merging solutions. The concatenated object is always a ComputedProcessedVariable, since converting to a ProcessedVariable is not viable - it requires access to the full state vector, which is unavailable when output_variables is used.

Fixes #4743

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• Tests added that prove fix is effective or that feature works