Clear_costmap_recovery - Different parameter syntax

The old behavior was to specify a string parameter, and any obstacle layer with that string within its name would be cleared. 

The new behavior is to specify a list of string names, and any costmap layer with a name that matches one on the list is cleared. 

Fix for #211. Possible fix for #198. 

Includes tests!
Perhaps change this to "Recovery behavior will clear layer %s" or something like that, so users have some idea what "using" means?