ENH: Add TriangularMesh data structure [BIAP9]

[effigies]

#1251 Added Pointset and Grid. #1090 also proposes TriangularMesh, but with the refactoring seen in #1251, that seemed like a bit of extra work for minimal benefit to the transformations use-case. This PR re-adds triangular meshes.

Overall, the approach of multiple names feels clunky and pushes the concerns of surface families into pointsets. #1251 removed that, and this PR extends this by keeping TriangularMesh focused on the base data structure and moves TriangularMeshFamily logic into a CoordinateFamilyMixin class that permits swapping out coordinates with a .with_name() method (open to bikeshedding). Hence, we would change:

left_surfaces.get_coords(name='pial')

to:

left_surfaces.with_name('pial').get_coords()

It also follows the lead of Grid and gets rid of the omni-__init__ in favor of targeted factory classmethods.

---

Notes from previous discussion:

• @oesteban wrote: "As a note, if some time we support other meshes with more than three edges, then n_triangles should be better named n_faces."
  • This was part of early discussions of BIAP9. IIRC the universality of triangular meshes makes them worth special-casing for now. FreeSurfer has old formats that use non-triangular meshes, and both FreeSurfer and we convert them to triangular on load:

    nibabel/nibabel/freesurfer/io.py

    Lines 145 to 168 in 590b226

    	if magic in (QUAD_MAGIC, NEW_QUAD_MAGIC): # Quad file
    	nvert = _fread3(fobj)
    	nquad = _fread3(fobj)
    	(fmt, div) = ('>i2', 100.0) if magic == QUAD_MAGIC else ('>f4', 1.0)
    	coords = np.fromfile(fobj, fmt, nvert * 3).astype(np.float64) / div
    	coords = coords.reshape(-1, 3)
    	quads = _fread3_many(fobj, nquad * 4)
    	quads = quads.reshape(nquad, 4)
    	#
    	# Face splitting follows
    	#
    	faces = np.zeros((2 * nquad, 3), dtype=int)
    	nface = 0
    	for quad in quads:
    	if (quad[0] % 2) == 0:
    	faces[nface] = quad[0], quad[1], quad[3]
    	nface += 1
    	faces[nface] = quad[2], quad[3], quad[1]
    	nface += 1
    	else:
    	faces[nface] = quad[0], quad[1], quad[2]
    	nface += 1
    	faces[nface] = quad[0], quad[2], quad[3]
    	nface += 1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.22%. Comparing base (4e7ad07) to head (368c145).
⚠️ Report is 640 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1257      +/-   ##
==========================================
+ Coverage   92.19%   92.22%   +0.02%     
==========================================
  Files          99       99              
  Lines       12458    12496      +38     
  Branches     2560     2565       +5     
==========================================
+ Hits        11486    11524      +38     
  Misses        648      648              
  Partials      324      324              

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[effigies]

This is simple but perhaps a little clunky. We could also do:

Suggested change

	def add_coordinates(self, name, coordinates):
	self._coords[name] = coordinates
	def add_coordinates(self, *args, **kwargs):
	self._coords.update(dict(*args, **kwargs))

This would allow it to take any inputs that would generate a dict, which might be a bit more ergonomic. I'm not 100% on this, so I haven't made the change.