[core] Graph: initial refactoring of graph loading API and logic

* API
Instead of having a single `load` function that exposes in its API
some elements only applicable to initializing a graph from a templates,
split it into 2 distinct functions: `load` and `initFromTemplate`.
Apply those changes to users of the API (UI, CLI), and simplify Graph
wrapper classes to better align with those concepts.

* Deserialization
Reduce the cognitive complexity of the deserizalization process
by splitting it into more atomic functions, while maintaining the
current behavior.

meshroom/core/typing.py

@@ -0,0 +1,8 @@
+"""
+Common typing aliases used in Meshroom.
+"""
+
+from pathlib import Path
+from typing import Union
+
+PathLike = Union[Path, str]