extractSlice( axis, index ) {

		const slice = new VolumeSlice( this, index, axis );
		this.sliceList.push( slice );
		return slice;

	}
Returns a slice corresponding to the given axis and index.
The coordinate are given in the Right Anterior Superior coordinate format.

@param {('x'|'y'|'z')} axis - The normal axis to the slice.
@param {number} index - The index of the slice.
@returns {VolumeSlice} The extracted slice.