DOC : clarified doc of add_artist

Added note about add_artist not automatically updating
the data limits used by the auto-scaling.
New animation example inspired from the Joy Division's Unchained Love cover
bug fix: check the type of the 'key' of the two array 'r1' and 'r2'
Merge pull request #3108 from tacaswell/add_artist_doc2

DOC : clarified doc of add_artist
Error message written in proper English
Merge pull request #3111 from montefra/mlab_bug

bug fix: check the type of the 'key' of the two array 'r1' and 'r2'
Merge pull request #3099 from rougier/unchained

New animation example (Joy Division's Unchained Love cover)