analysis: generate sheet relation diagram about Merger/Scatter

[wenchy]

message Diagram {
    map<string, Book> books = 1; // book name -> Book
    message Book {
        string alias = 1; // alias
        map<string, Sheet> sheets = 2; // sheet name -> Sheet
    }
    message Sheet {
        string alias = 1; // alias
        Primary primary = 2;
    }
    message Primary {
        string book_name = 1;
        string sheet_name = 2;
        RelationType relation_type = 3; 
    }
    enum RelationType {
        RELATION_TYPE_MERGER = 1;
        RELATION_TYPE_SCATTER = 2;
    }
}