[cleanup] Don't zero-initialize matrix in local assemblers

Zero-initilization already happens in the global assembler. Hence you
don't need to and in fact should not do this in the local one, too.
This has the advantage, that you can chain local assembler. E.g. you
can get a local assembler for the H^1 norm using

```cpp
auto massAssembler = Dune::Fufem::MassAssembler();
auto laplaceAssembler = Dune::Fufem::LaplaceAssembler();
auto h1Assembler = [=](auto&&... args) {
  massAssembler(args...);
  laplaceAssembler(args...);
};
```

or by implementing an abstract utility

```cpp
auto sumLocalAssembler = [](auto... localAssemblers) {
  return [=] (auto&&... args) {
    std::initializer_list<int> { (localAssemblers(args...), 0)...};
  };
};
auto h1Assembler = sumLocalAssembler(Dune::Fufem::MassAssembler(), Dune::Fufem::LaplaceAssembler());
```