[Solver] Update examples to use the new StationaryLinearProblemSolver constructor

The StationaryLinearProblemSolver constructors were cleaned up to always
take the backend before the DOF vector. Make sure all examples use the
new version.
[Cleanup] Remove unused typedefs and variables
[Cleanup] Remove / update include statements
[CMake] Deactivate broken dnaplmfd example
[Adaptivity] Use adapt_grid() version that explicitly specifies the integration order
[dgdiffusion-hp] Replace leafView() -> leafGridView()
[ldomain] Remove deprecated dimension parameter from PkFEM typedef
[Cleanup] GenericReferenceElements have been dead for a long time...
[Whitespace] Clean up whitespace across all files recently touched to fix warnings