get_slice for nifti subclass for 4D to 3D image

[lucywowen]

for next release, change nifti get_slice method to something like this:

def get_slice(self, index):
if len(self.shape)>3:
if self.shape[3]==1:
return nib.Nifti1Image(header=self.header, dataobj=np.squeeze(self.get_data()), affine=self.affine)