Task.wait_for_valid_timestamp doesn't return the timestamp

[bkeryan]

DAQmxWaitForValidTimestamp returns the timestamp: https://www.ni.com/docs/en-US/bundle/ni-daqmx-c-api-ref/page/daqmxcfunc/daqmxwaitforvalidtimestamp.html

So does Task.WaitForValidTimestamp in .NET: https://www.ni.com/docs/en-US/bundle/ni-daqmx-net-framework-40-api-ref/page/ninetdaqmxfx40ref/html/m_nationalinstruments_daqmx_task_waitforvalidtimestamp.htm

And so do the interpreter classes: https://github.com/ni/nidaqmx-python/blob/master/generated/nidaqmx/_library_interpreter.py#L5831

But Task.wait_for_valid_timestamp does not return anything: https://github.com/ni/nidaqmx-python/blob/master/generated/nidaqmx/task/_task.py#L1035

Cc: @DeborahOoi96

[bkeryan]

Fixed by #592