function addLayerClass (elem) {
  const classes = elem.getAttribute('class')
  if (!classes || !classes.length) {
    elem.setAttribute('class', Layer.CLASS_NAME)
  } else if (!Layer.CLASS_REGEX.test(classes)) {
    elem.setAttribute('class', classes + ' ' + Layer.CLASS_NAME)
  }
}
Add class `Layer.CLASS_NAME` to the element (usually `class='layer'`).

@param {SVGGElement} elem - The SVG element to update
@returns {void}