Write plan of action for `keep_compatible`

[thomas-qah]

@noracato and @mabijkerk recently discussed the current functionality of the keep_compatible-flag in etengine and possible improvements for this. An overview document and the following issues came forth from this:

quintel/etengine#1361 quintel/etengine#1362 quintel/etmodel#4145 quintel/etmodel#4146

Before these will be implemented we need further discuss the details.

[thomas-qah]

Done! Outcome:

Ready to be picked up/implemented:

• 1361,

• quintel/etmodel#4145

Still need discussion (maybe next sprint?):

• 1362,

• quintel/etmodel#4146

Futhermore, @noracato and I defined a new concept to better track scenario migrations, whether they affected certain scenario's, and how or why in this PR: #1364