Add initial root hash in Deep Subtree

[Manav-Aggarwal]

A Deep Subtree should be able to be initialized with an initial root hash since the root is set to nil for an empty Deep Subtree. However, if a non-nil root exists in the Deep Subtree, it should have priority over the initial root hash and the working hash of the Deep Subtree should be used as the root hash.

Add setters and getters for initial root hash along with using it in verifyOperation and buildTree.

Parent issue: #20