API: Closes #7879: (drops not nan in panel.to_frame() by default)
closes #7879 by changing API (filter_observations=True now deprecated and replaced with dropna=False)

this will need a sub-section in whatsnew/0.17.0. showing what the original behavior is and what the new is. 
 can you update according to comments
 @jreback: updated the commit according to your comments!
 - need a test for the deprecation itself.
- need a separate sub-section in the whatsnew explaining this change
 added/fixed tests and added sub-section
 this is pushed to 0.18.0 pls update when you can
 closing, but if you'd like to update, pls reopen
