Template FiniteElement over scalar type

[garth-wells]

We have an issue that functions like FiniteElement::apply_inverse_transpose_dof_transformation call operations, such as multiplication, on scalar and double data. However, some operations are not defined, e.g. multiplication of a double and a std::complex<float>. We haven't see this issue before with Basix because PETSc does some magic in its header files to permit these operations.