class_docstr: This is a class to represent multiple ROI objects, where the
  reference to a given domain is explicit.
  
  A multiple ROI object is a set of ROI defined on a given domain,
  each having its own 'region-level' characteristics (ROI features).
  
  Every voxel of the domain can have its own characteristics yet,
  defined at the 'voxel-level', but those features can only be accessed
  familywise (i.e. the values are grouped by ROI).
  
  Parameters
  ----------
  k : int
    Number of ROI in the SubDomains object
  label : array of shape (domain.size), dtype=np.int_
    An array use to define which voxel belongs to which ROI.
    The label values greater than -1 correspond to subregions
    labelling. The labels are recomputed so as to be consecutive
    integers.
    The labels should not be accessed outside this class. One has to
    use the API mapping methods instead.
  features : dict {str: list of object, length=self.k}
    Describe the voxels features, grouped by ROI
  roi_features : dict {str: array-like, shape=(self.k, roi_feature_dim)
    Describe the ROI features. A special feature, `id`, is read-only and
    is used to give an unique identifier for region, which is persistent
    through the MROI objects manipulations. On should access the different
    ROI's features using ids.
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docstr: Return a voxel-wise feature, grouped by ROI.
  
  Parameters
  ----------
  fid: str,
    Feature to be returned
  id: any hashable type
    Id of the ROI from which we want to get the feature.
    Can be None (default) if we want all ROIs's features.
  
  Returns
  -------
  feature: array-like, shape=(roi_size, feature_dim)
    if an id is provided,
       or list of arrays, shape=(roi_size, feature_dim)
    if no id provided (default)
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code: def get_feature(self, fid, id=None):
      """Return a voxel-wise feature, grouped by ROI.
  
      Parameters
      ----------
      fid: str,
        Feature to be returned
      id: any hashable type
        Id of the ROI from which we want to get the feature.
        Can be None (default) if we want all ROIs's features.
  
      Returns
      -------
      feature: array-like, shape=(roi_size, feature_dim)
        if an id is provided,
           or list of arrays, shape=(roi_size, feature_dim)
        if no id provided (default)
  
      """
      if fid not in self.features:
          raise ValueError(f"the `{fid}` feature does not exist")
      if id is not None:
          feature = np.asarray(self.features[fid][self.select_id(id)])
      else:
          feature = self.features[fid]
      return feature
  
imports: import numpy
  from nibabel import Nifti1Image, load
  from nipy.io.nibcompat import get_affine, get_header
  from .None import discrete_domain
label_desc: Use image node_id 11 for calling the SubDomains obj's underlying member method code with example usage: obj.get_feature(fid, id) and returns: feature