Results 3 issues of xywei

This is the first of several PRs to refactor the tree build. This PR adds the very basic classes and methods to enable generation of uniform box mesh for `volumential`...

This is the continuation of #35, adding adaptive tree generation functionalities.

Type inference on the following kernel fails with message `UnboundLocalError: local variable 'symbols_with_unavailable_types' referenced before assignment`: ```python import pyopencl as cl import numpy as np import loopy as lp ctx...