randomized svd draft (#3008)

* randomized svd
add windows feature
return range sketch matrix Q
support for subset of samples and individuals in a tree
lint requires a function inside a loop to be explicit about the loop variable; fixed it by setting a default variable
fix docs build error; add PCAResult to python-api.md

Author: hanbin973

docs/python-api.md

@@ -1590,3 +1590,9 @@ Also see the {ref}`sec_python_api_metadata` summary.
     :members:
     :private-members: _repr_svg_
 ```
+
+#### The {class}`PCAResult` class
+```{eval-rst}
+.. autoclass:: PCAResult
+	:members:
+```