docs(example/base_registrar.py): add example implementation for SpeckleBase class register

re #50

example/base_registrar.py

@@ -0,0 +1,37 @@
+"""This is an example module for automated base model subtype registration."""
+from pydantic import BaseModel
+from typing import ClassVar, Dict, Type
+
+
+class ExampleBase(BaseModel):
+    foo: int
+
+    _registry : ClassVar[Dict[str, Type["ExampleBase"]]] = {}
+
+    def __init_subclass__(cls, speckle_type:str, **kwargs):
+        # this requires some validation, there is nothing blocking identical keys...
+        cls.speckle_type = speckle_type
+        ExampleBase._registry[speckle_type] = cls
+        super().__init_subclass__(**kwargs)
+
+
+class ExampleSub(ExampleBase, speckle_type="sub"):
+    bar: str
+
+
+class SpeckleSub(ExampleSub, speckle_type="speckle_sub"):
+    magic : str
+
+
+# # uncommenting this raises an error, for missing speckle_type definition
+# # this automatically enforces mandatory type definition for everyone
+# class FaultySub(ExampleBase):
+    # pass
+
+
+if __name__ == "__main__":
+
+    for k,v in ExampleBase._registry.items():
+        print(f"Speckle type {k}, mapped to {v.__name__}")
+
+    print(ExampleBase._registry["speckle_sub"](foo=123, magic="trick", bar="baric").json())