Slow mode assumes time units in seconds

[matthew-d-jones]

Slow mode assumes event_time_zero datasets and NXlog time datasets have units of seconds.
If event_time_zero is not in seconds then the stream will appear to start but hang.

Note that publishing sample environment also has a very crude implementation which assumes a constant 10Hz source:

NeXus-Streamer/nexus_file_reader/src/NexusFileReader.cpp

Line 147 in 54bdaee

auto frameNumber = static_cast<size_t>(std::floor(time / 0.1));