Load WVec[] from MiniYaml

Will be used by #6100 for loading sub-cell info from mod.yaml
This includes a few test cases (in one commit) to be removed before merging.
Newlines for each && case would look better.
```csharp
if (WRange.TryParse(parts[3 * i], out rx) &&
    WRange.TryParse(parts[3 * i + 1], out ry) &&
    WRange.TryParse(parts[3 * i + 2], out rz))
```