See #1437 for background. While it would be ideal to allow for the complete
emptying of a Tree, the root node is required (and it's part of the
construction of a Tree). So, here, when clearing the Tree we optionally
allow for a new label to be given.
Ideally we'll also allow for fresh data to be provided too; but there's a
wrinkle there in knowing the difference between the data being None, and no
data being provided (so the current root's data being carried over).
Following the method of defaulting used in __init__ would cause problems. As
such, rather than roll all of this into one commit, this goes with the basic
requirement and the solution for data will follow.
Note this also starts some unit tests for the clearing of a Tree.
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