add example: convection with plates

[mulvrova]

Add data to Example directory: convection in spherical annulus featuring plates at the surface. In addition add an example of surface velocity plotting in the cookbook. Size of the new directory should not exceed a few MB.

[amorison]

A solution to make it light would be to include only a few (like 3) snapshots, and drop stuff like rprof and time series. StagyyData is robust against missing data, so this wouldn't prevent its use.