Some features in this megre request

• Using dune-functions GlobalBasis
• Assembling loops over elements first, then over nodes in basis-tree
• Operators can be associated to nodes in basis-tree
• Scalar and Vector operators
• DOFVectors are global coefficient vectors (for flat indices in the globalBasis)
• Various operators implemented for zot, fot, sot

Issues:

• not all expressions are available, e.g. no gradientOf
• not block structure in the matrices any more. This needs an extension in the problem-traits generator and an automatic generation of SystemVector and SystemMatrix data-structures. Maybe extend only for one-level blocks