Define two layers for observations: new and historical

[damianooldoni]

This PR implements the two layer strategy for observations as mentioned in #204:

• historical observations (from 2020-01-01 up to 2024-06-04)
• new observations (from 2024-06-05 onwards)

The new obs are shown immediately by default. The historical can be activated by user via checklist in layer panel.

At the moment the circles are identical for both layers.