Add option for acquisition system to supply session start and reference times

`session_start_time` and `timestamps_reference_time` are set in `NWBFile.cpp` to the current time. 

```c++
  io->createStringDataSet("/session_start_time", time);
  io->createStringDataSet("/timestamps_reference_time", time);
```

We may want to add the option for the acquisition system to supply those times as parameters.