Add options for TPE and example, grad test.

[MTCam]

This is the minimal lift that allows users/developers to use TPE easily in mirgecom drivers. How-to use is demonstrated by the combozzle example, and a correctness test for TPE is added to the gradient test in test_operators.py.

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[tulioricci]

Just wondering if we should keep pulse as it is and only add pulse-tpe instead.

Other than that, LGTM.