`step_size` of `iterate` method is not accurate

[ariostas]

When you pass memory sizes into the various iterate methods you end up getting arrays with sizes that are quite different to the size that you requested. It happens for both TTrees and RNTuples, but it is much worse for RNTuples.

This was first reported by @pfackeldey when studying memory usage and fragmentation.

I'm looking into this first, and then follow up with a more careful look into the usage and fragmentation.