Missing CreateGroupIssueBoard function

Created by: paulvollmer

Hello,

it looks like the func to create group issue boards is missing for https://godoc.org/github.com/xanzy/go-gitlab#GroupIssueBoardsService here are the gitlab api docs: https://docs.gitlab.com/ce/api/group_boards.html#create-a-group-issue-board-premium

or maybe i don't find the right go func to create a group board. if the func is missing, i can implement it if you like.

thanks in advance, paul