Duplicate equations

[MarcoArtiano]

At the moment we have two set of equations for each formulation of the compressible Euler equations, one with gravity and one without. The second set of equations is equivalent when initializing $$\phi$$ with zero.

We should consider removing this duplication. That depends how we will handle in the future terms that are not evolved in time such as the geopotential (see add auxiliary PR).

[sloede]

If you make gravity optional, you should consider making the decision about whether to include gravity or not in the type signature of the equations struct, such that you can use zero-cost dispatch on whether gravity is activated or not.