Add iso element on hexahedron (#684)

* Added iso element on hexahedron

* update README

* avoid segfaults

* fixes

Author: mscroggs

README.md

@@ -151,6 +151,7 @@ The following elements are supported on a hexahedron:
   - [Bubble](https://defelement.com/elements/bubble.html)
   - [DPC](https://defelement.com/elements/dpc.html)
   - [Serendipity](https://defelement.com/elements/serendipity.html)
+  - [iso](https://defelement.com/elements/p1-iso-p2.html)
 
 
 ### Prism

cpp/basix/e-lagrange.cpp

@@ -851,7 +851,7 @@ create_d_iso(cell::type celltype, int degree, element::lagrange_variant variant,
     _M(i, 0, i, 0) = 1.0;
 
   return FiniteElement(
-      element::family::P, celltype, polyset::type::macroedge, degree, {},
+      element::family::iso, celltype, polyset::type::macroedge, degree, {},
       impl::mdspan_t<const T, 2>(math::eye<T>(ndofs).data(), ndofs, ndofs),
       impl::to_mdspan(x), impl::to_mdspan(M), 0, maps::type::identity,
       sobolev::space::L2, true, degree, degree, variant,
@@ -1434,10 +1434,11 @@ FiniteElement<T> basix::element::create_iso(cell::type celltype, int degree,
                                             lagrange_variant variant,
                                             bool discontinuous)
 {
-  if (celltype != cell::type::interval && celltype != cell::type::quadrilateral)
+  if (celltype != cell::type::interval && celltype != cell::type::quadrilateral
+      && celltype != cell::type::hexahedron)
   {
     throw std::runtime_error("Can currently only create iso elements on "
-                             "intervals and quadrilaterals");
+                             "intervals, quadrilaterals, and hexahedra");
   }
 
   if (variant == lagrange_variant::unset)
@@ -1575,7 +1576,7 @@ FiniteElement<T> basix::element::create_iso(cell::type celltype, int degree,
   auto tensor_factors
       = create_tensor_product_factors<T>(celltype, degree, variant);
   return FiniteElement<T>(
-      family::P, celltype, polyset::type::macroedge, degree, {},
+      family::iso, celltype, polyset::type::macroedge, degree, {},
       impl::mdspan_t<T, 2>(math::eye<T>(ndofs).data(), ndofs, ndofs), xview,
       Mview, 0, maps::type::identity, space, discontinuous, degree, degree,
       variant, dpc_variant::unset, tensor_factors);