Vertex-only primitives

[fsimonis]

Main changes of this PR

This PR simplifies triangles, by defining them in terms of vertices instead of edges.
This reduces their size in memory and avoids having to remap edge IDs in filter and communication operations.
It also simplifies handling triangles in tests.
The change is internal only and has no observable side effects in the API.

Major change is that triangles can now exist independent of edges.

In short:

• No breaking change
• lower memory footprint
• less communication while sending meshes
• faster filtering and sending of meshes
• A triangle ABC doesn't imply the existence of edges AB, BC, and AC any longer.

Motivation and additional information

See #1153

Author's checklist

• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I ran make format to ensure everything is formatted correctly.
• I sticked to C++14 features.
• I sticked to CMake version 3.16.3.
• I squashed / am about to squash all commits that should be seen as one.

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?

[uekerman]

I did not look at every detail, but I think I understand the changes and the reasoning 👍

It also simplifies handling triangles in tests.

which is not yet part of this PR, right?

[fsimonis]

which is not yet part of this PR, right?

@uekerman no, this will require changing many tests.