constructor(parent, groupDef) {
        this.TYPE = 'group'
        this.name = groupDef.name
        this.parent = parent
        this.group = groupDef
        this.id = this.group.id
        this.g = this.group.g
        this.env = this.group.env
        this._env = {}
    }
Create a Group object.
@param {[type]} parent     The parent flow/group
@param {[type]} groupDef   This group's definition