_animateIn(index) {
      let $collapsibleLi = this.$el.children('li').eq(index);
      if ($collapsibleLi.length) {
        let $body = $collapsibleLi.children('.collapsible-body');

        anim.remove($body[0]);
        $body.css({
          display: 'block',
          overflow: 'hidden',
          height: 0,
          paddingTop: '',
          paddingBottom: ''
        });

        let pTop = $body.css('padding-top');
        let pBottom = $body.css('padding-bottom');
        let finalHeight = $body[0].scrollHeight;
        $body.css({
          paddingTop: 0,
          paddingBottom: 0
        });

        anim({
          targets: $body[0],
          height: finalHeight,
          paddingTop: pTop,
          paddingBottom: pBottom,
          duration: this.options.inDuration,
          easing: 'easeInOutCubic',
          complete: (anim) => {
            $body.css({
              overflow: '',
              paddingTop: '',
              paddingBottom: '',
              height: ''
            });

            // onOpenEnd callback
            if (typeof this.options.onOpenEnd === 'function') {
              this.options.onOpenEnd.call(this, $collapsibleLi[0]);
            }
          }
        });
      }
    }
Animate in collapsible slide
@param {Number} index - 0th index of slide