Make FEAssembler store a const reference of the basis

... rather than a copy.  This allows to assemble with bases
that are not copyable, as for example the RaviartThomasBasis

See https://gitlab.dune-project.org/staging/dune-functions/-/issues/58

CHANGELOG.md

@@ -4,6 +4,9 @@
 - Introduce class `LocalIntegralEnergy` to work with the densities
 - Local energies and FE assemblers use now `dune-functions` power bases instead of scalar `dune-fufem` bases; the key element is the `LocalView` which contains the information for each element
 - Introduce class `LocalHyperDualStiffness` and `hyperdual` to calculate gradient and hessian using hyper-dual numbers
+- The class `FEAssembler` now stores a `const` reference to the basis, rather than a value.
+  This allows to use the assembler with bases that are not copyable
+  (as, e.g., the `RaviartThomasBasis`, see [dune-functions issue 58](https://gitlab.dune-project.org/staging/dune-functions/-/issues/58)).
 
 ## Deprecations and removals
 

dune/elasticity/assemblers/feassembler.hh

@@ -33,7 +33,7 @@ class FEAssembler {
 
 public:
 
-    const Basis basis_;
+    const Basis& basis_;
 
     /** \brief Partition type on which to assemble
      *