[operators] some renaming
* DarcyReconstruction -> Darcy
* ReconstructionOperator::DiffusiveFlux
  -> Operators::DiffusiveFluxReconstruction
* ProjectionOperator::Lagrange -> Operators::LagrangeProjection
* ProjectionOperator::L2 ->Operators::L2Projection
* ProjectionOperator::Generic -> Operators::Projection
* ProjectionOperator::Dirichlet -> Operators::DirichletProjection
* ProlongationOperator::Lagrange -> Operators::LagrangeProlongation
* ProlongationOperator::L2 ->Operators::L2Prolongation
* ProlongationOperator::Generic -> Operators::Prolongation