Prevent users from changing values of certain attributes

It would be a nice feature to be able to create attributes with values that cannot be edited by the user when instantiated.

For example, a class Alfa Romeo with a curb weight of 895 kg. This value should not be editable by the user when the class is instantiated.

It is unclear how to add this to AToMPM, and it would be a highly invasive change.

The issue names no files, tests, or entry points, so first map how AToMPM defines attributes and handles user edits during instantiation. Clarify the expected model and attribute semantics before implementation. Done means an attribute can be created with an initial value that users cannot change afterward, without unintended effects on other attributes.

Written by the indexing model from the issue text.