Ignore NaN warnings in angle helper.

It it expected that the array may contain nan and this is handled correctly using nanmin
Filter invalid warning in calculation of theta
Filter warning in tight layout demo:
Reenable plots in tight layout demo
Hexbin, Dont calculate mean of empty slice

Avoids numpy warnings with the same result. Similar to what is done above
Merge branch 'travis_docs_deploy'
Merge branch 'examples_filter_warnings'