Implement a class for diagrams (collections of cells (i,j) indexed by the natural numbers), and a class for diagrams with the northwest property. A diagram has the northwest property if the fact that (i1, j1) and (i2,j2) are in the diagram implies that the cell (min(i1,i2), min(j1,j2)) are in the diagram.

Depends on #34339
Depends on #34343
Depends on #34345

CC: @tscrim

Component: combinatorics

Keywords: gsoc2022 northwest-diagram

Author: Trevor K. Karn

Branch/Commit: 382926c

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/34260