Add time stamp to data log file

[conorrussomanno]

Here's all I changed to log the system time:

Inside dataFiles.pde:

public void writeRawData_dataPacket(DataPacket_ADS1299 data, float scale_to_uV, float scale_for_aux) {
if (output != null) {
writeTime();
output.print(Integer.toString(data.sampleIndex));
writeValues(data.values,scale_to_uV);
writeValues(data.auxValues,scale_for_aux);
output.println(); rowsWritten++;
//output.flush();
}
}

private void writeTime() {
output.print(String.format("%d", System.currentTimeMillis()));
output.print(", ");
}

Thanks very much for all the help!