Rejected years and POT QMED analysis

Handles rejected years in AMAX records (note the data flag) and calculates QMED from POT data.

The POT QMED analysis does take into account data gaps, but doesn't ensure address the seasonality of the data buy filing in gaps to crease complete years. TODO.

Note that loading of the NRFA data takes longer as it also loads all POT files. We might want to make this optional although loading of the data only takes place once (and at updates of NRFA dataset).

Note also that the NRFA POT dataset is not very clean. There are occurrence of 2 peaks falling on the same date and data gaps having the same start date.