Add Heightmap class

[chapulina]

Resolves #382.

Added a C++ API for heightmap geometries that matches the XML spec.

I don't plan on changing the PR, but I'd like to see it working with ign-gazebo before merging, so I'll keep it as a draft while we work on the other parts.

[scpeters]

looks ok to me; I'll look into how to add it to ign-physics; it will probably be similar to how meshes are handled, using ign-common

[chapulina]

I'll look into how to add it to ign-physics; it will probably be similar to how meshes are handled, using ign-common

In case it helps, this is how I'm using common::HeightmapData in ign-rendering.

[chapulina]

I'm working on the ign-gazebo PR and I have the need for some API to add textures and blends to a heightmap object. We don't do that much in the SDF DOM API, the only place where I saw Add functions is in sdf::Actor. Given that people have been discussing the API design (#292), are there new guidelines for this use case?

If it's still undecided, I guess I could leave this as it is and use the Element API to construct an object instead...

[chapulina]

This PR is working with gazebosim/gz-sim#487. I still have some details to fix on that one, but I think the API on this one is settled, so I'm opening it for review, @scpeters.

On a related note, because ign-rendering doesn't directly depend on SDF, and I needed a class there to hold heightmap data, I ended up making an almost exact copy of sdf::Heightmap for ignition::rendering::HeightmapDescriptor on gazebosim/gz-rendering#180. It's worth mentioning that similar data is also present in ignition::msgs::HeightmapGeom. I ticketeted an issue about all this duplication with some ideas: gazebosim/gz-sim#494

[codecov-io]

Codecov Report

Merging #388 (1b737a5) into sdf9 (0bca996) will increase coverage by 0.24%.
The diff coverage is 97.64%.

@@            Coverage Diff             @@
##             sdf9     #388      +/-   ##
==========================================
+ Coverage   86.43%   86.68%   +0.24%     
==========================================
  Files          60       61       +1     
  Lines        9304     9516     +212     
==========================================
+ Hits         8042     8249     +207     
- Misses       1262     1267       +5     

Impacted Files	Coverage Δ
src/Geometry.cc	89.38% <61.53%> (-3.62%)	⬇️
src/Heightmap.cc	100.00% <100.00%> (ø)

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