Improve validation of PDS trips

[efcaguab]

The information obtained from the PDS trips is used to calculate the most important statistics from Timor. Unfortunately, the quality of the data is patchy.

For instance, the model we're currently using indicates that the average boat makes 330-360 landings per year. This is very likely to be not the case and might be caused by trips in the system that are not accurate.

We need to identify these spurious trips using more stringent automated checks than we are currently use.

[langbart]

@efcaguab there are actually several trips that last less than an hour. I think the best thing is to identify them by adding a lower threshold and merge those that have relatively short intervals between end and start in the same trip. What do you think?