Reduce duplication by moving some overloads to the base class.

Optimised build was omitting XZInterpolation factory registrations.

* next: (194 commits)
  invert_laplace: Fix outer boundary metrics
  Apply clang-format changes
  Apply clang-format changes
  Apply suggestions from code review
  Fix missed adios
  Prefer ADIOS2 over ADIOS in more cases
  Update tests/unit/test_extras.hxx
  Apply clang-format changes
  Silence warning about assignment in `if` statement
  Fix some short identifiers
  Make a couple of implicit casts explicit
  CI: Suppress some warnings about short identifiers
  Move some static functions to anonymous namespace
  CI: Remove deprecated clang-tidy option
  Include header for `BoutReal`; remove unused header
  Apply clang-format
  Add options to set Butcher tables by name
  Simplify SUNDIALS cmake
  Switch to [[maybe_unused]]
  Apply suggestions from code review
  ...

Based on @dschwoerer's implementation in #2651

Currently gives pretty bad results, not sure why

Although `getGlobal` is only valid for local indices, unfortunately
`isLocal` calls `getGlobal`, so we can't actually check!