Add checks that the key is not bigger than maximum key length for the tree
maxLevels size, where maximum key len = ceil(maxLevels/8).

This is because if the key bits length is bigger than the maxLevels of the
tree, two different keys that their difference is at the end, will collision in
the same leaf of the tree (at the max depth).

- Update public methods signatures
- Remove 'lastAccess' param
- Add option to pass root for tree.Dump, Iterate, IterateWithStop,
Graphviz (and related)
- Move error messages to const defined error messages for external usage

CASE C: ALMOST CASE B --> if Tree has few Leafs (but numLeafs>=minLeafsThreshold)
==============================================================================
- Use A, B, G, F as Roots of subtrees
- Do CASE B for each subtree
- Then go from L to the Root

              R
             /  \
            /    \
           /      \
          *        *
         / |      / \
        /  |     /   \
       /   |    /     \
L:    A    B   G       D
              / \
             /   \
            /     \
           C      *
                 / \
                /   \
               /     \
              ...    ... (nLeafs >= minLeafsThreshold)