Extend AeroForces filter to compressible flows
This MR extends FilterAeroForces to compressible flows. This was done by storing a (weak) pointer to the equation system creating the filter (the same had to be done for forcings), allowing the filter to call auxiliary virtual functions implemented in each solver (GetVelocity, GetPressure, GetDensity). Any filter using these auxiliary functions will automatically support IncNS and CFS, eliminating the need to implement a workaround for CFS in each case. I also applied this idea to the energy filter, which no longer needs derived classes inside each solver.
Also, now FilterFieldConvert calls ExtraFldOutput for getting extra variables for the CFS (Mach, pressure,...). This is especially important when using FilterAverageFields (which is derived from FilterFieldConvert), since otherwise it would not be possible to obtain the time-average of these quantities.