[mesh] Refactor MeshTaichi API

[g1n0st]

Issue: #3608

This PR is targeting for officially releasing MeshTaichi. For simplifying the usage and deprecating out-dated design decisions, we would like to update MeshTaichi front-end APIs:

Current:

import taichi

mesh = ti.TriMesh()
mesh.verts.place({'x' : ti.math.vec3})
mesh.faces.link(mesh.verts)
model = mesh.build("bunny.obj")

Updated:

import taichi
import meshtaichi_patcher as Patcher

mesh = Patcher.load_mesh("bunny.obj", relations=['FV'])
mesh.verts.place({'x' : ti.math.vec3})

Full updates:

• Remove implicit data type cast warnings
• Add deprecate warnings for out-dated APIs
• Support new APIs & add documentations
• Remove (experimental) automatic relation analysis
• Add full warnings for undeclared mesh relations
• Expose get_position_as_numpy() instead of implicit data copy

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[BillXu2000]

LGTM