Allow reassigning and resetting AbstractContainer attributes/fields

[rly]

Motivation

Fix #411, fix #865

Major change to allow setting attributes on Container objects to new values, including None. If the attributes were read from a file (i.e. container_source is not None), then a warning is raised.

Note that we currently do a lot of validation at the constructor level (type validation, shape validation) in docval. We do not do that for each setter. This can result in errors on write if the user is not careful. We could instead move validation of attributes to outside of docval and into the setter. That is significantly more development, but I think worthwhile in the long run. I will open a separate issue for this.

Checklist

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