/// <summary>
 /// Initializes a new instance of the <see cref="FixedEulerStateDerivative"/> class.
 /// </summary>
 /// <param name="method">The method.</param>
 public FixedEulerStateDerivative(FixedEuler method)
 {
     _method = method.ThrowIfNull(nameof(method));
     _index  = method.StateManager.AllocateState(1);
     _states = method.IntegrationStates;
 }
 /// <summary>
 /// Initializes a new instance of the <see cref="FixedEulerStateDerivative"/> class.
 /// </summary>
 /// <param name="method">The method.</param>
 public FixedEulerStateDerivative(FixedEuler method)
 {
     _method = method;
     _index  = method.StateManager.AllocateState(1);
     _states = method.IntegrationStates;
 }