remove MultiIndexTree from core

Within the rearrangement process, it became clear, that there is a conceptional issue with the MultiIndexTree and its attachment to Grid. Since the MultiIndexTree is highly related to the interpolation schemes, it shall be found in minterpy.schemes. However, this is not possible, since in the current status, grid.tree is used in minterpy.schemes.barycentric, and can actually not be removed easily. This is also the reason, why dds.py stays on top level and is not moved to minterpy.schemes.divided_differences, because this would cause some weird circular imports.

Idea: maybe we shall reimplement MultiIndexTree as a subclass of Grid adding the functionality needed (and removing the grid.tree attribute), and then replace grid.tree in the functions of minterpy.schemes.barycentric with the new subclass.

This is somewhat related to #18