fix(panel): don't throw exceptions when the groupName is a string
fix(panel): don't throw exceptions when the groupName is a string

- the APIs take a string or Array, we need to handle that w/o exceptions
- fix invalid JSDoc/Closure syntax
- remove duplicate header comments

oostmeijer/material,angular/material,bluebirdtech/material,marekmicek/material,angular/material,marekmicek/material,oostmeijer/material,bluebirdtech/material,angular/material,oostmeijer/material,bluebirdtech/material,bluebirdtech/material,oostmeijer/material,angular/material,marekmicek/material,marekmicek/material