Clarified type refinement, simplified API, custom exception for get_pointer

Type refinement is now only done with BlendFileBlock.refine_type(), and
no longer with sdna_index_refine parameters to various functions. This
simplifies the API at the expense of having to call two simple functions
instead of one more complex one.

blender_asset_tracer/blendfile/iterators.py

@@ -0,0 +1,9 @@
+from . import BlendFileBlock
+
+
+def listbase(block: BlendFileBlock) -> BlendFileBlock:
+    """Generator, yields all blocks in the ListBase linked list."""
+    while block:
+        yield block
+        next_ptr = block[b'next']
+        block = block.bfile.find_block_from_address(next_ptr)