print runtime stats to the environment variable EMPER_STATS_FILE

• Make all stats print methods accept a std::ostream as output.
• Move the printing of runtime component stats intoRuntime::printStats.
• Use Runtime::printStats instead of Runtime::printLastRuntimeStats in ~Runtime, because we are already in a runtime which may differ from Runtime::currentRuntime.
• Write the stats in ~Runtime to a possible file passed in the environment variable EMPER_STATS_FILE