getUrlFromAttr = function (attrVal) {
  if (attrVal) {
    // url('#somegrad')
    if (attrVal.startsWith('url("')) {
      return attrVal.substring(5, attrVal.indexOf('"', 6))
    }
    // url('#somegrad')
    if (attrVal.startsWith("url('")) {
      return attrVal.substring(5, attrVal.indexOf("'", 6))
    }
    if (attrVal.startsWith('url(')) {
      return attrVal.substring(4, attrVal.indexOf(')'))
    }
  }
  return null
}
Extracts the URL from the `url(...)` syntax of some attributes.
Three variants:
 - `<circle fill="url(someFile.svg#foo)" />`
 - `<circle fill="url('someFile.svg#foo')" />`
 - `<circle fill='url("someFile.svg#foo")' />`
@function module:utilities.getUrlFromAttr
@param {string} attrVal The attribute value as a string
@returns {string} String with just the URL, like "someFile.svg#foo"