Revised getitem operation for masked jagged indexers

[nsmith-]

First pass, some new kernels will need to be made.

[jpivarski]

Thanks a lot! I just have a lot of little comments.

[nsmith-]

I think the functionality is there now. I think the boilerplate for kernels is pretty out of hand, would be nice to find a way to reduce it. Remaining items:

• How do I auto-format to 80 characters?
• Where do I put some documentation of this behavior?
• Why is the doctest failing

[jpivarski]

• How do I auto-format to 80 characters?

We don't have a tool for that; I did it by hand. If it's roughly or mostly 80 characters, that is fine.

• Where do I put some documentation of this behavior?

__getitem__ behavior is documented here: https://github.com/scikit-hep/awkward-1.0/blob/cfa3a04d679529175e742fb5015047bd499ea1a6/src/awkward1/highlevel.py#L507-L843

• Why is the doctest failing

Merge upstream master; it was fixed today (PR #323).

[jpivarski]

I'm going to merge your branch with master, which should fix the doctest.

After that, it's done, right? You're not planning any more changes?

[jpivarski]

We're going to ignore the doctest error; I'll ask Reik to look into that after all the merges are done.

Assuming all the other tests pass, should I merge this? I'm dealing with the compiler warnings on another PR.

[nsmith-]

Ok, now that its formatted and docs are added, I think its ready

[nsmith-]

Closes #315