Add barotropic fluid

Implementent a variational solver for the barotropic compressible fluid model as a third step towards non-linear MHD.

The new model is VariationalBarotropicFluid in models.toy, it relies on two propagators : VariationalMomentumAdvectionand VariationalDensityEvolve

The first propagator implements a Crank-Nicolson step for self-advection term in euler equation,

\int_{\Omega} \partial_t \rho u \cdot v - \rho u \cdot [u,v] dx = 0

And was already implemented in the previous MR !442 (merged)

And the second one a Crank-Nicolson step for the density advection and corresponding term in the momentum equation

\int_{\Omega} \partial_t \rho u \cdot v + \big( \frac{|u|^2}{2} - \frac{\partial \rho e}{\partial \rho} \big) \nabla \cdot (\rho v) dx = 0

\partial_t \rho + \nabla \cdot (\rho u) = 0

where e is the internal enegy, here e=\rho/2. I modify the already existing VariationalDensityEvolve to allow choosing the model and avoid creating another very similar propagator.

Closes #234 (closed)