docstr: Splits the array of parameters in x_f to 'x' and 'f' for performing
  a search on the both of them independently using the Trust Region
  Method.
  
  Parameters
  ----------
  x_f : array
      Combined array of parameters 'x' and 'f' parameters.
  
  Returns
  -------
  x, f : array
      Split parameters into two separate arrays
code: def x_f_to_x_and_f(self, x_f):
      """
      Splits the array of parameters in x_f to 'x' and 'f' for performing
      a search on the both of them independently using the Trust Region
      Method.
  
      Parameters
      ----------
      x_f : array
          Combined array of parameters 'x' and 'f' parameters.
  
      Returns
      -------
      x, f : array
          Split parameters into two separate arrays
  
      """
      x = np.zeros(2)
      f = x_f[0]
      x = x_f[1:3]
      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 7 for calling the IvimModelVP obj's underlying member method code with example usage: obj.x_f_to_x_and_f(x_f) and returns: x,  f