Start drafting PatchIntersectionMap.

[drwells]

Part of #159.

After looking at #179 I came up with some ideas for how we can efficiently store and iterate over the intersections. At a high level we can build on fdl::PatchMap: once we know which elements intersect which patches we can compute all the intersections. For starters I am just doing one intersection per stencil.

Some next steps:

• [x] fill out the Accessor class. This has the advantage of putting all of the difficult to get right SideIndex calculations in one place.
• [x] write a corresponding iterator class
• [x] write some tests in which we manually populate the internal fields of PatchSingleIntersections

It should be possible to do most of this class without actually writing the real intersection code. That can come later (and it is logically in a level below this one).

[drwells]

More remains to be done here but this will give us a nice way to access single intersections.

[ryanleaf1000]

Hello David, any suggestions for next step?

[drwells]

Please don't post questions in closed PRs - this is a discussion that should happen in #159. Lets plan things out there.