docstr: Calculates the fractional anisotropy.
          
docstr_tok: ["Calculates","the","fractional","anisotropy","."]
code: def fractional_anisotropy(self):
      r"""Calculates the fractional anisotropy."""
      fa = np.sqrt(
          0.5
          * (2 * (self.d_par - self.d_perp) ** 2)
          / (self.d_par**2 + 2 * self.d_perp**2)
      )
      return fa
  
imports: from warnings import warn
  import numpy
  from dipy.reconst.cache import Cache
  from dipy.reconst.multi_voxel import multi_voxel_fit
  from dipy.reconst.csdeconv import csdeconv
  from dipy.reconst.shm import real_sh_descoteaux_from_index
  from scipy.special import gamma, hyp1f1
  from dipy.core.geometry import cart2sphere
  from dipy.data import default_sphere
  from dipy.reconst.odf import OdfModel, OdfFit
  from scipy.optimize import leastsq
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 3 for calling the ForecastFit obj's underlying member method code with example usage: obj.fractional_anisotropy() and returns: fa