Restructure energy filter to support CFS

This MR:

- Restructures the energy filter into an abstract class so that only specific solvers are supported (i.e. those that support the concept of kinetic energy/enstrophy)
- Adds implementations for the incompressible and compressible solvers
- Adds appropriate documentation and tidies existing filter documentation
- Removes uses of `\checkmark` in the docs in deference to `\cmark` which is already used elsewhere and has a corresponding `\xmark`.

See merge request !388