More keyword constructors for volume/surface terms

This came up in https://github.com/trixi-framework/Trixi.jl/pull/695#discussion_r670550426. It would be nice to have keyword constructors such as

- `SurfaceIntegralWeakForm(surface_flux=flux_central)`
- `SurfaceIntegralStrongForm(surface_flux=flux_central)`
- `VolumeIntegralFluxDifferencing(volume_flux=flux_central)`

to make it easier to change some parameters via `trixi_include`.

Start by locating the definitions of SurfaceIntegralWeakForm, SurfaceIntegralStrongForm, and VolumeIntegralFluxDifferencing, then inspect how trixi_include passes parameters to them. Add the requested keyword constructor forms and verify that surface_flux and volume_flux can be changed through trixi_include without altering other settings.

Written by the indexing model from the issue text.