docstr: Initialize a IvimFit class instance.
  
  Parameters
  ----------
  model : Model class
  
  model_params : array
      The parameters of the model. In this case it is an
      array of ivim parameters. If the fitting is done
      for multi_voxel data, the multi_voxel decorator will
      run the fitting on all the voxels and model_params
      will be an array of the dimensions (data[:-1], 4),
      i.e., there will be 4 parameters for each of the voxels.
code: def __init__(self, model, model_params):
      """Initialize a IvimFit class instance.
  
      Parameters
      ----------
      model : Model class
  
      model_params : array
          The parameters of the model. In this case it is an
          array of ivim parameters. If the fitting is done
          for multi_voxel data, the multi_voxel decorator will
          run the fitting on all the voxels and model_params
          will be an array of the dimensions (data[:-1], 4),
          i.e., there will be 4 parameters for each of the voxels.
  
      """
      self.model = model
      self.model_params = model_params
  
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 1 to create a new IvimFit object  with example: obj = IvimFit(model, model_params)