Allow BlockCollector to split blocks into depth-1 layers

[chriseclectic]

What should we add?

Would it be possible to add to BlockCollector.collect_all_matching_blocks the ability to optionally split the collected blocks into layers of depth 1? Currently it can only split based on disjoint qubit sets.

For an example, imagine collecting all blocks of 2 qubit gates, and then splitting them into layers of depth 1. Eg I might have a non-split block like (represented by barriers below):

      ░                 ░ 
q_0: ─░───■─────────■───░─
      ░ ┌─┴─┐     ┌─┴─┐ ░ 
q_1: ─░─┤ X ├──■──┤ X ├─░─
      ░ └───┘┌─┴─┐└───┘ ░ 
q_2: ─░───■──┤ X ├──■───░─
      ░ ┌─┴─┐└───┘┌─┴─┐ ░ 
q_3: ─░─┤ X ├─────┤ X ├─░─
      ░ └───┘     └───┘ ░ 

and want it to be separated into multiple blocks of depth-1

      ░       ░       ░       ░ 
q_0: ─░───■───░───────░───■───░─
      ░ ┌─┴─┐ ░       ░ ┌─┴─┐ ░ 
q_1: ─░─┤ X ├─░───■───░─┤ X ├─░─
      ░ └───┘ ░ ┌─┴─┐ ░ └───┘ ░ 
q_2: ─░───■───░─┤ X ├─░───■───░─
      ░ ┌─┴─┐ ░ └───┘ ░ ┌─┴─┐ ░ 
q_3: ─░─┤ X ├─░───────░─┤ X ├─░─
      ░ └───┘ ░       ░ └───┘ ░