[forms][cleanup] Severe cleanup and performance improvements of multilinear operators

This collects many improvements of the Dune::Fufem::Forms multilinear operators. Most importantly it simplifies and unifies the interface across 0-, 1-, and 2-linear operators and makes code more readable.

This also improves performance of the operator assembler - in part dramatically (see (*) below). Especially for the linear-elasticity hyperelasticity examples assembly is improved by a factor of 3-4.

Detailed list of changes in this MR:

• Multilinear operators:
  • Extract common parts of differential operators into a common base class.
  • Unify and strip down interface and exported typedefs.
  • Improve/add some static assertions.
• Tensor implementations (results of evaluating operators)
  • Extract tensor operation into Dune::Fufem::Impl::Tensor::.
  • Improve interface of LazyTensorBlock and bring it closer to std::mdspan.
  • Add special tensor implementation for dyadic products.
  • When adding a 2-tensor to a local matrix exploit the dyadic product to avoid duplicate evaluation of differential operators for the same basis function (*).