Automatically determine the measurement interval

Instead of using the somewhat arbitrary values for measurement interval in each PMT implementation, measure is at runtime in PMT::SetMeasurementInterval(int milliseconds), when milliseconds == 0. This function is automatically called in PMT::StartThread() to get an appropriate measurement interval. Users can adjust the interval by passing another value to this function.

The dump interval (which could be set using the PMT_DUMP_INTERVAL) environment variable is now in milliseconds, because it makes sense to dump using a custom interval faster than once every second.