Implement DG Cartoon partial derivatives

[michaeldmurphy1]

Proposed changes

The first commit is to allow a Mesh to take a Cartoon basis which can only ever have one grid point.

The second commit uses Michael Pajkos's Cartoon code to find partial derivatives not in the computational domain. As we divide by the coordinates, if there is a zero value, L'Hopital's rule is used to evaluate the derivative at that point.

Within PartialDeriatives.tpp, cartoon_contraction() does the contractions in Eqn. (218) of the SXS book's Numerical Method's chapter, specifically the thing on the RHS that you take the derivative of or divide by x. The cartoon_derivative() function does the bottom choice of Eqn. 218 (dividing by x) while the top choice is evaluated if needed at the bottom of cartoon_partial_derivatives_apply(). One only ever calls cartoon_partial_derivatives().

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

[nilsdeppe]

Really good work on this! This is very technical code and I think you did a fantastic job. I have quite a few suggestions, but many are stylistic and you'll just get used to them :)

[nilsdeppe]

LGTM, a few minor typos, please rebase on develop, squash your commits appropriately, and also squash the minor changes immediately (no need to have a separate fixup commit for them)