Proposed changes

Useful to look at mesh.extents() in ParaView without having to write a kernel file.

Just do this:

spectre transform-vol path/to/volfiles*.h5 -k spectre.Spectral.Mesh3D:extents

Then run generate-xdmf and open in ParaView.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments