Clean up `bp_get_group_avatar()`:

* Remove `$bp` global
* Use `bp_parse_args()`
* Code format tidying
* Pass `$r` through filter
* Add documentation

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8751 cdf35c40-ae34-48e0-9cc9-0c9da1808c22