Make schemes safe against user changes

[hagenw]

At the moment a user can change the dtype or labels of a scheme, but this will not automatically update the related tables.
So we should make those attributes properties that return a copy and provide setter functions. BTW, you can add a setter function to a Python property, so a user should be able to do something like schemes['my-scheme'].labels[0] = 'a' if we want to allow for it.

[frankenjoe]

At some point we have introduced Scheme.replace_labels() to offer the user a safe way to replace labels. This does of course not solve the issue that a user might do schemes['my-scheme'].labels[0] = 'a' which is not safe. However, it is not straightforward to rename the attribute labels to e.g. _labels and instead introduce a property labels as in that case it would not be stored to YAML any longer. So I wonder if we should simply stick with Scheme.replace_labels() as the proper way to change labels, just like a user should e.g. not directly modified Table.df.

[hagenw]

I agree now that we have Scheme.replace_labels() users should no longer be tempted to replace labels directly.
I would also propose to not do anything here.
I mark this issue as wontfix for now.