Add warning sign for keep_compatible default

[mabijkerk]

Currently, if users create a scenario using the API, the attribute keep_compatible is set to false by default. We want to keep this default setting, but there is a risk that it's overlooked by relevant users (both internal and external). This can lead to issues like quintel/etengine#1393.

The goal therefore is to add some kind of warning message that informs users that keep_compatible is set to false.

• Determine a fitting warning message
• Determine whether this message should be include for both the scenario tools and the API in general or just the scenario tools
• Implement the new warning message