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.