Save cached markers for later re-publish

[artivis]

Hi, first of thanks for the great tool !

While doing some debugging I was troubled by the fact that after disabling/re-enabling a marker (marker namespace) in Rviz, the marker would not show up again.

It seems that by design the trigger clears the cached markers. Thus calling trigger in a while loop is pointless unless the markers are fed again. I circumvent this the following manner :

bool RvizVisualTools::trigger(const bool keep_cache) // keep_cache is false by default
{
...
if (!keep_cache)
    markers_.markers.clear();  // remove all cached markers
  return result;
}

/* in main */
while (ros::ok())
{
    // Don't forget to trigger the publisher! And save the cache!
    visual_tools_->trigger(true);

    ros::spinOnce();
    rate.sleep();
}

I'd be happy to open a PR, just wanted to check first with you if that was an appropriate design.
Maybe you'd prefer more explicit functions calls like :

visual_tools_->keepCache(true);

// Don't forget to trigger the publisher!
visual_tools_->trigger();

// with helpers
bool cache_keeped = visual_tools_->isKeepingCache();
visual_tools_->clearCache();

Cheers.