MultiCut to store multi-channel recordings with shared supervision

[desh2608]

This PR adds a MultiCut which is a new type of Cut that represents multi-channel data. We also define a DataCut as a Cut which contains actual data (as opposed to a PaddingCut or MixedCut which are empty or operations on other cuts). This DataCut is the abstract base class from which MonoCut and MultiCut inherit.

 >>> 2-channel telephone recording with 2 supervisions, one for each channel (e.g., Switchboard):

                  ╔══════════════════════════════  MultiCut  ═════════════════╗
                  ║ ┌──────────────────────────┐                              ║
     Channel 1  ──╬─│   Hello this is John.    │──────────────────────────────╬────────
                  ║ └──────────────────────────┘                              ║
                  ║                               ┌──────────────────────────┐║
     Channel 2  ──╬───────────────────────────────│ Hey, John. How are you?  │╠────────
                  ║                               └──────────────────────────┘║
                  ╚═══════════════════════════════════════════════════════════╝

   
>>> Multi-array multi-microphone recording with shared supervisions (e.g., CHiME-6),
    along with close-talk microphones (A and B are distant arrays, C is close-talk):

               ╔═══════════════════════════════════════════════════════════════════════════╗
               ║ ┌───────────────────┐                         ┌───────────────────┐       ║
       A-1   ──╬─┤                   ├─────────────────────────┤                   ├───────╬─
               ║ │ What did you do?  │                         │I cleaned my room. │       ║
       A-2   ──╬─┤                   ├─────────────────────────┤                   ├───────╬─
               ║ └───────────────────┘  ┌───────────────────┐  └───────────────────┘       ║
       B-1   ──╬────────────────────────┤Yeah, we were going├──────────────────────────────╬─
               ║                        │   to the mall.    │                              ║
       B-2   ──╬────────────────────────┤                   ├──────────────────────────────╬─
               ║                        └───────────────────┘        ┌───────────────────┐ ║
        C    ──╬─────────────────────────────────────────────────────┤      Right.       ├─╬─
               ║                                                     └───────────────────┘ ║
               ╚════════════════════════════════  MultiCut  ═══════════════════════════════╝

[desh2608]

I think this one's ready for review once the tests pass.

[csukuangfj]

 >>> 2-channel telephone recording with 2 supervisions, one for each channel (e.g., Switchboard):

                  ╔══════════════════════════════  MultiCut  ═════════════════╗
                  ║ ┌──────────────────────────┐                              ║
     Channel 1  ──╬─│   Hello this is John.    │──────────────────────────────╬────────
                  ║ └──────────────────────────┘                              ║
                  ║                               ┌──────────────────────────┐║
     Channel 2  ──╬───────────────────────────────│ Hey, John. How are you?  │╠────────
                  ║                               └──────────────────────────┘║
                  ╚═══════════════════════════════════════════════════════════╝

The visualization looks great!

[pzelasko]

Wow, very impressive work!! I left just a few comments, mostly minor ones + asking for a bit more test coverage, otherwise it looks great!

[desh2608]

@pzelasko Addressed all comments.

[pzelasko]

LGTM! Again, great work.