Use float64 as key in bptree

[nrkfeller]

As it currently stands. It seems I can only use "types.Int" as keys in the bptree is there a way we can use float64 instead?

[timtadh]

@nrkfeller you just need to implement the types.Hashable interface for float. I haven't implemented it because I haven't needed it yet. I just did a quick implementation in #2 . Please test that out and let me know if works. If you wanted to write a test that would be great as well but do not feel obligated.

[nrkfeller]

Beautiful! I will write tests when I have a bit of time. Thank you!