Release v6.0

Key new features in this release

• more modularity by allowing lambda functions in most dg classes in particular matrix inversion, multigrid and timesteppers. Create new solvers by freely combining existing ones or link to extermal libraries with a small lambda wrapper
• Allow std::map < key, Container> in blas1 functions
• the new dg/matrix/matrix.h library for matrix-function computations
• new improved timestepper interface that allows purely implicit solvers in implicit timesteppers

Break backwards compatibility

• dg::norm was removed (finally) -> fix all compile errors by simply removing all occurences of dg::normed and dg::not_normed
• dg::PCG now works with self-adjoint matrices in given weights instread of symmetrics ones
• implicit and semi-implicit timesteppers now expect a separate solver instance -> use std::tie to tie parameters together and dg::DefaultSolver to create the usual PCG time solve