Bugfix: Tree Capacity

[llucid-97]

Hi, I've been playing with your repo trying to get a PER implementation working.
I notice the SumTree 's get() method doesn't retrieve data correctly unless the capacity is a power of two because of this logic in the _retrieve method:

        left = 2 * idx + 1
        right = left + 1

        if left >= len(self.tree):
            return idx

if the tree capacity is not a power of two, the recursion terminates exits at the wrong index (either going past or stopping before where it should).
I've added an assertion to check for this, and a bit of logic to make it easy to set the capacity without having to worry about this