Advanced testing for external features

A conclusion of #393 (closed) is that external features should be tested with fake data before launching the chain in order to minimize the risk of having a crash due to bad user input. I'm opening a new issue to keep concerns separate.

There are several strategies:

• create a new class with methods like get_interpolated_Sentinel2_B4 providing fake data
• pass fake data to the existing class with an OTB pipeline

A crash in the fake execution would be caught and printed to the user with a stack trace allowing to debug the function.