Harvest absences

[damianooldoni]

In order to show absences (see #271) we need first to collect such data. The best approach would be to harvest absences from GBIF. But, to do so these null observations for both Cambaridae and Astacidae must flow to GBIF.

@fjasteen will investigate this with Natuurpunt. In particular, she will check if Natuurpunt can publish these absences. Based on the last agreement for IAS data publication, they do not published any absence. Hopefully they can do it in the context of this project: hope, we all know, is the last to die.

If this is not possible, we can collect these data ourselves but it could be cumbersome.