improve frost views

The current frost views are getting increasingly slower with growing data.

We need to improve the performance by reworking the sql queries and/or strategically defining some views as materialized views. If we use materialized views we need to think about a process to refresh them.

The affected views are:

• "Observation"
• "FeaturesOfInterest"
• "Datastreams" due to "observed_area" needing to process all Features of the datastream
• some of the frost helper views