docstr: Combines the array of parameters 'x' and 'f' into x_f for performing
  NLLS on the final stage of optimization.
  
  Parameters
  ----------
   x, f : array
      Split parameters into two separate arrays
  
  Returns
  -------
  x_f : array
      Combined array of parameters 'x' and 'f' parameters.
code: def x_and_f_to_x_f(self, x, f):
      """
      Combines the array of parameters 'x' and 'f' into x_f for performing
      NLLS on the final stage of optimization.
  
      Parameters
      ----------
       x, f : array
          Split parameters into two separate arrays
  
      Returns
      -------
      x_f : array
          Combined array of parameters 'x' and 'f' parameters.
  
      """
      x_f = np.zeros(3)
      x_f[0] = f[0]
      x_f[1:3] = x
      return x_f
  
imports: import warnings
  import numpy
  from scipy.optimize import least_squares, differential_evolution
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.multi_voxel import multi_voxel_fit
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 8 for calling the IvimModelVP obj's underlying member method code with example usage: obj.x_and_f_to_x_f(x, f) and returns: x_f