We now check that a file is at least a few seconds old
before attempting to import it.
The actual minimum age can be configured in etc/config.yaml or
else is defaults to 10 seconds.
The idea is that this should give the OS enough time to fully
write the file before we import it.
The timestamp being looked at is the modification time.
Fixes#92.
We do this so that we can look for the "saillineOffset"
parameter, which we expect to be present in source
preplot imports and allows us to correlate source
and sail lines.
The change to bin/sps.py is necessary to let the JSON
serialisation take place.
