Refactoring imagelists

Usecases for ImageList

Thus far only used in anger in nipy.modalities.fmri.fmristat.model, similarly in nipy.modalities.fmri.spm.model.

From that file, an object obj of class FmriImageList must:

• return 4D array from np.asarray(obj), such that the first axis (axis 0) is the axis over which the model is applied
• be indexable such that obj[0] returns an Image instance, with valid shape and coordmap attributes for a time-point 3D volume in the 4D time-series.
• have an attribute volume_start_times giving times of the start of each of the volumes in the 4D time series.
• Return the number of volumes in the time-series from len(obj)