Container item delete methods

Currently there is no documented obvious way to remove existing NIX entities.

When looking through the code, one can see, that there are `__delitem__` methods on the custom container classes, so items can be removed either by accessing these magic functions or by using the `del` keyword, but this is not obvious to the average user.

Is there a reason for not providing a `remove` or `delete` method on `Container` instances?

Review the custom container classes and their existing __delitem__ methods, which the issue identifies as the current removal entry points. Determine whether a remove or delete method should be added and what its API should guarantee; the issue is complete only when that behavior and its user-facing documentation are decided and implemented.

Written by the indexing model from the issue text.