Read session start time from header

[ageorgou]

We've had a FIXME comment in the code for this, but it's now becoming relevant in one of the tests too!

Each NWB file must have a session start time. Normally, we get this from the speed data. Where that is not available, we use the current time. Howeve, the start time may also be recorded in the header, so we could use that instead of the current time. As a side-effect, this will give a consistent signature for the test.

Even when the speed data is available, it is possible for the header to have a different experiment time recorded (as in this comment). Which one should take precedence in those cases?