Update the paper milestone

[rouson]

@gutmann Feel free assign issue #19 and #20 to @scrasmussen and adjust the paper milestone to have a July 31 deadline and you can probably close issue #26 (or at least remove it from the paper milestone) because presumably @afanfa won't have time to work on it.

Possibly there could also be a paper project set up via GitHub for gathering issues into a kanban board, but it's also possible that's overkill in this case. Your call.

[gutmann]

Created a project (because I'm trying to test that functionality too) https://github.com/gutmann/coarray_icar/projects/1

Removed #26 from paper milestone
I don't think I can assign issues to @scrasmussen until he contributes to something (even an issue comment?)