Add function to create a spacetime tensor from time and spatial ones.

[ncorsobh]

Proposed changes

The function combines the desired time component of a tensor with the desired spatial components of a tensor to produce one spacetime tensor. For instance, it could combine a time derivative with a spatial derivative to produce a spacetime derivative.

If there are locations throughout the code where this could be usefully applied to replace existing code, please suggest them!

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

[wthrowe]

Looks good. Squash.