## New features to compressible solver

This MR includes a few new features to the CFS:

- A forcing term
`Quasi1D`

used to obtain a 1D Euler simulation with area variation (quasi-1D nozzle). - Another forcing
`AxiSymmetric`

for axi-symmetric Euler (the symmetry axis is x=0). In this case it should also be possible to perform a 3D simulation in a 2D mesh (by using`<GEOMETRY DIM="2" SPACE="3">`

) to obtain an axi-symmetric flow with swirl. - A new boundary condition type
`StagnationInflow`

for prescribing stagnation density and pressure, which is useful in nozzle flows. - It is now possible to perform local time-stepping based on each element CFL by setting
`<I PROPERTY="LocalTimeStep" VALUE="True"/>`

. This could accelerate the convergence to a steady-state. - An explicit filtering technique to stabilize the flow (only in 1D and 2D), activated using
`<I PROPERTY="ExponentialFiltering" VALUE="True"/>`

. - It is now possible to change the reduction in P used to calculate the artificial diffusion sensor (using the parameter
`SensorOffset`

). This is useful because the filtering of the previous item zeroes the highest mode, and therefore we can only combine the filter with the artificial diffusion using SensorOffset=2.

I tried to include the most important items of this list in the user guide, but I guess some parts are still missing.

This also fixes a series of bugs:

- IProductWRTDerivBase was not working when spacedim!=expdim
- DG in one dimension was not working in parallel
- The non-smooth artificial viscosity was wrong when Kappa!=0, and also the denominator of the sensor
- The initial condition chk was being overwritten by the first checkpoint (only in the CFS)
- CompressibleFlowSystem::GetStdVelocity was wrong, affecting the CFL calculation
- In the fld output, the sensor was replacing the pressure result.