Create alias for elements

[massimiliano-leoni]

I ran into this shortcoming while elaborating on the hyperelasticity demo.
There, in the UFL/py file, we export an expression sigma

expressions = [(sigma, [[0.25, 0.25, 0.25]])]

and then create a function space for it via basix with

const basix::FiniteElement S_element =
        basix::create_element(family, cell_type, k, variant, discontinuous);
auto S = std::make_shared<fem::FunctionSpace>(fem::create_functionspace(
                 problem.getMesh(),
                 S_element,
                 pow(problem.getMesh()->geometry().dim(), 2)));

Now, since saving a DG0 tensor function is currently unsupported [see https://gitlab.kitware.com/vtk/vtk/-/issues/18458 and https://github.com/FEniCS/dolfinx/issues/2298 ] but -- and I say BUT -- writing a DG1 tensor works with ADIOS2, I thought I'd do that.
Unfortunately, this doesn't work because basix only creates elements with a value shape of {} [see https://github.com/FEniCS/basix/issues/626 ] and then dolfinx mistakenly assigns a vector value shape

// dolfinx/fem/FiniteElement.cpp:266
if (_value_shape.empty() and bs > 1)
  _value_shape = {1};
std::transform(_value_shape.cbegin(), _value_shape.cend(),
               _value_shape.begin(), [bs](auto s) { return bs * s; });        // is {9}, should be {3, 3}

and ultimately, down the line,

// dolfinx/io/ADIOS2Writers.cpp:91
const int rank = u.function_space()->element()->value_shape().size();         // is 1, should be 2
const std::uint32_t num_comp = std::pow(3, rank);                             // is 3, should be 9

which eventually yields a segfault in ADIOS2.

So fine, I'll export the function space from UFL instead of creating it with basix:

DG1T = TensorElement("DG", tetrahedron, 1)
elements = [(DG1T)]

but then DG1T is nowhere to be found in the generated code. I see that the element is being exported to the C file but there is no alias or handle to instantiate it.
Ideally I would expect to be able to do something like

auto S = std::make_shared<fem::FunctionSpace>(
        fem::create_functionspace(functionspace_DG1T, "", mesh));