check whether we wanna allow assigment on task (measure, etc) properties

[berndbischl]

at least in the "weights PR" properties became an AB. where assignment was possible.
I don't like that. props should be read-only? as static as possible?

[mb706]

I agree that allowing assignment to properties may be a bad idea here; it is not entirely clear what 'removing' a property should do. E.g. for weights: remove the associated col roles? Or just signal that the relevant col role should be ignored? For missings: remove features with missing values?