Supress some warnings in examples

This filters a number of warnings from the examples. 

*  Numpy invalid values which are already handled correctly. 
*  Warnings about tight layout which is handled manually in the example
*  Explicitly handle mean of empty slices to avoid warnings. 

I have done this for 2 reasons

1.  I don't think that end users should be seeing warnings when running examples. That will lead to confusion for end users. 
2.  The warnings are raised when building the documentation. Any real issues might be concealed by these warnings. 
Right you are. I guess I was being lazy :smiley: 
I assume that np.unroll() should we np.unwrap?