Feature/add executed on time check (#7)

* Initial commit on executed on time feature. A task was executed on time if the function call happened within one second since it expired.

* Adding tests, fixing some errors.

* Using recursirve mutex to allowing to call safely call was_executed_on_time in an Mt-environment

* Changing from boolean expression to get_delay, being even more flexibel

* Cleanup

* Adding dedicated TaskContext

* Changing to Interface-Class Approach

* Renaming to TaskInformation, making it pure virtual

* Removing unnecessary Proxy-Class

* Cleaning up

* Passing a const reference instead of a pointer to avoid nullptr checks in the callback

* Cleaning up add_schedule.

* Adding TaskInformation API to readme.

Co-authored-by: Heinz-Peter Liechtenecker <h.liechtenecker@fh-kaernten.at>

README.md

@@ -8,7 +8,7 @@ Libcron offers an easy to use API to add callbacks with corresponding cron-forma
 ```
 libcron::Cron cron;
 
-cron.add_schedule("Hello from Cron", "* * * * * ?", [=]() {
+cron.add_schedule("Hello from Cron", "* * * * * ?", [=](auto&) {
 	std::cout << "Hello from libcron!" std::endl;
 });
 ```
@@ -23,6 +23,28 @@ while(true)
 }
 ```
 
+The callback must have the following signature:
+
+```
+std::function<void(const libcron::TaskInformation&)>
+```
+
+`libcron::Taskinformation` offers a convenient API to retrieve further information:
+
+- `libcron::TaskInformation::get_delay` informs about the delay between planned and actual execution of the callback. Hence, it is possible to ensure that a task was executed within a specific tolerance:
+
+```
+libcron::Cron cron;
+
+cron.add_schedule("Hello from Cron", "* * * * * ?", [=](auto& i) {
+	using namespace std::chrono_literals;
+	if (i.get_delay() >= 1s)
+	{
+		std::cout << "The Task was executed too late..." << std::endl;
+	}
+});
+```
+
 
 
 ## Removing schedules from `libcron::Cron`