frequency interface is used to publish
a frequency to other objects.
The get method returns the current frequency.
The object implementing this interface is expected to implement
the Sim_Notify_Frequency_Change notifier, which should be
raised when the object changes its frequency. Objects subscribing
to this notifier can acquire the new frequency by doing a new call
to the get method.
SIM_INTERFACE(frequency) {
double (*get)(conf_object_t *NOTNULL obj);
};
#define FREQUENCY_INTERFACE "frequency"