gradients for euler 2d and swe 2d

[fnrizzi]

• including the header for euler2d and swe2d automatically includes the gradients

• gradients are NOT computed automatically by the problem/app object for several reasons:

  • the gradient just needs a mesh object and field/function on it, so it is fully orthogonal to what the field is/comes from. Indeed, an app object never owns the state nor the mesh.
  • decoupling the two makes things more flexible for how and when the gradients are calculated
  • decoupling also allows to contruct the gradient evaluator independently of the app object

• test added only for Euler2d for now

• drive-by changes to fix template for gradient class and test