vlt.stats.power.getLMESimFunc

  GETLMESIMFUNC - Selects the correct simulation function handle.
    This helper function acts as a router. Based on the user's 'Method'
    selection ('gaussian', 'shuffle', or 'hierarchical'), it returns a
    function handle to the appropriate simulation function. This allows the
    main loop in `lme_power_effectsize` to call the selected simulation
    method using a consistent syntax via `feval`.