[ENH] add constructor to merge BlockIndexRange #467
Description
Consider 2 BlockIndexRange{1} b1 = Block(1)[1:1] and b2 = Block(3)[2:5]. There is currently no simple way to merge them into Block(1,3)[1:1,2:5], one has to write Block((Block(b1), Block(b2)))[only(b1.indices), only(b2.indices)]. A dedicated constructor would be very useful.