Make the MultiBlock API saner

This is just to track our discussion in https://github.com/pyvista/pyvista/pull/1482#discussion_r664009578:

This is a pretty confused API 😦

1. blocks.keys() is a thing, and instantiating from a list comes up with string keys, so it looks like a dict.
2. Iterating over the container, however, yields values, which is not like a dict.
3. There’s no blocks.values() nor blocks.items()…
4. blocks[i] supports both keys and integer indices?!

Additionally, I’ve also noticed that MultiBlock__setitem__ does this:

import pyvista as pv

blocks = pv.MultiBlock()
blocks[3, 'name'] = pv.Sphere()

blocks.keys()
# [None, None, None, 'name']

Making a 2-tuple passed to __setitem__ act neither like a list nor like a dict is especially problematic from an API design standpoint.

I’m still not sure if we can change any of this without breaking too much downstream code, but at least we can track this future work on this issue.

[MatthewFlamm]

You are right, I’m focusing first on __setitem__/__getitem__ in this list. I wanted to start writing down the current behavior in one place. Feel free to add directly to my list or start a new list for other behaviors.

[adeak]

I know you probably just mean get/setitem here, but multi.keys() and iter(multi) behaviour will need thought too.