IdpfCache implementation using a binary tree

[divergentdave]

Chris suggested adding an IdpfCache implementation that uses a binary tree, as the cached values are arranged in a tree already. Note that this will have to sparsely populated, because the full binary tree may get impractically large.

[cjpatton]

I suspect this will be the most efficient option for VIDPF.

[armfazh]

In #978, I implemented a binary tree for this purpose.