Add accelerator

[hgangwar]

This PR adds the transient coupling accelerator interface and an initial Aitken relaxation implementation.

• Accelerator: defines the interface for updating coupling iterates.
• AitkenAccelerator: dynamically adjusts the relaxation factor using changes between successive interface residuals.
• BeginWindow: resets iteration history at the start of each coupling time window.
• Unit tests verify the initial relaxation, Aitken update, and window reset behavior.

Usage

  pcms::transient::AitkenAccelerator accelerator(0.5);

  accelerator.BeginWindow();

  std::vector<double> x_in{1.0, 2.0};
  std::vector<double> x_out{2.0, 4.0};

  accelerator.Update(x_in, x_out);

Here, x_in is the current interface iterate and x_out is the interface result produced by the coupled solve. After Update, x_in contains the relaxed iterate for the next fixed-point iteration.

N.B.: this PR needs to rebased once PR #380 is merged.