Add new observation models

This is a WIP

What does the code in the MR do ?

This MR continues the work of !97 (merged) and proposes to add missing Observation Models.

Details TBA...

Where should the reviewer start ?

TBA

How can the code be tested ?

TBA

When is the MR due for? (review deadline)

TBA

What issues are linked to the MR ?

TBA