Moving SGFS, CSG from pymc3 to pymc3-experimental (#3391)

* Moving SGFS, CSG from pymc3 to pymc3-experimental

* Removed Unused Imports

* Removed notebooks containing CSG and SGFS

* Updated Release Notes

Author: vbsinha

RELEASE-NOTES.md

@@ -31,6 +31,7 @@
 ### Deprecations
 
 - `nuts_kwargs` and `step_kwargs` have been deprecated in favor of using the standard `kwargs` to pass optional step method arguments.
+- `SGFS` and `CSG` have been removed (Fix for [#3353](https://github.com/pymc-devs/pymc3/issues/3353)). They have been moved to [pymc3-experimental](https://github.com/pymc-devs/pymc3-experimental).
 
 ## PyMC3 3.6 (Dec 21 2018)