Move Minmod implementation into a .tpp file

[fmahebert]

Proposed changes

• Move the implementation of the Minmod limiter from a .cpp file to a .tpp file. The extra work to "type erase" neighbor data so that it could be used in a template-free function in the .cpp file becomes unnecessary, and so is removed.
• Add explicit instantiations of the Minmod limiter for relevant systems (Burgers and ValenciaDivClean). These explicit instantiations should allow the compile-time implementation not to further bog down the compilation of executables.

Types of changes:

• Bugfix
• New feature### Further comments

Component:

• Code
• Documentation
• Build system
• Continuous integration

Code review checklist

• The PR passes all checks, including unit tests, clang-tidy and IWYU.
  For instructions on how to perform the CI checks locally refer to the Dev
  guide on the Travis CI.
• 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.

[nilsdeppe]

I just have one topic for design discussion, but over all I'm happy with this.

[nilsdeppe]

Do you think we should generalize this to LimiterInstantiations.cpp and instantiate all limiters there?

[fmahebert]

As I see it: the advantages to grouping the instantiations into a single file are (a) having fewer files and (b) making it easier to see what limiters are available for each system; the downside is having to re-instantiate all limiters when one limiter changes. Are there other considerations that I am missing?

At the moment I do not have a strong opinion on the matter, so I am happy to make the change if you prefer it. Just let me know what you think 🙂

[nilsdeppe]

Those are the only two things I can think of. Given that the number of limiters we will have will be < 10 (or at least given the current literature it seems unlikely to exceed 10) I think having separate files so that the compilation can be parallelized better is a good thing :)