Adding rate control for ConditionalRun function

Devices implementing the conditional run method are typically source
devices and a rate control can be desireable. New option '--rate' with
a float number argument in Hz can be used to configure rate control.
By default it is switched off.

fairmq/FairMQDevice.h

@@ -535,6 +535,8 @@ class FairMQDevice : public FairMQStateMachine
     bool fExternalConfig;
 
     const fair::mq::tools::Version fVersion;
+    float fRate;
+    size_t fLastTime;
 };
 
 #endif /* FAIRMQDEVICE_H_ */