HeaderlessTopicDiagnostic should clean up after itself

[poggenhans]

The HeaderlessTopicDiagnostic object registers a callback to the Updater class that is not removed, when HeaderlessTopicDiagnostic goes out of scope. The next update() then leads to a segfault.

Here is code to reproduce the issue:

using namespace diagnostic_updater;
Updater updater;
double min_freq{0.}, max_freq{std::numeric_limits<double>::infinity()};
auto diag = std::make_unique<TopicDiagnostic>( "topic_name", updater,  
                    FrequencyStatusParam(&min_freq, &max_freq, 0, 5), TimeStampStatusParam(0, 1));
updater.force_update(); // ok
diag.reset();
updater.force_update(); // segfault