Make read_meshtags a template function

[mleoni-pf]

This PR leverages functionalities introduced some time ago by @jorgensd that makes xdmf_utils::distribute_entity_data process data of a generic type, not only std::int32_t. In this PR I make XDMFFile::read_meshtags a template so that we can read mesh tags of any type [esp. double] from file. This will be useful in an upcoming PR to read functions from file.

[garth-wells]

What types are supported? If the type in the file can't be checked it can lead to all sorts of crashes and mis-behaviours.

What is needed for this to be robust is a function for querying the data type in the file. I started this ages ago for HDF5 files, but it wasn't trivial to make it precise, but getting the type size was easy and may be adequate.

[jhale]

We hit the same issue with the recent checkpointing project. We should have a chat in #developmemt about the best ways to tackle this.

[mleoni-pf]

I implemented something like this [untested]:

// Define a type-trait-kinda thing
template <typename T>
struct xdmf_data
{
};

template <>
struct xdmf_data<std::int32_t>
{
  static const std::string data_type;
  static const std::size_t precision = 4;
};
const std::string xdmf_data<std::int32_t>::data_type = "Int";

template <>
struct xdmf_data<std::int64_t>
{
  static const std::string data_type;
  static const std::size_t precision = 8;
};
const std::string xdmf_data<std::int64_t>::data_type = "Int";

template <>
struct xdmf_data<float>
{
  static const std::string data_type;
  static const std::size_t precision = 4;
};
const std::string xdmf_data<float>::data_type = "Float";

template <>
struct xdmf_data<double>
{
  static const std::string data_type;
  static const std::size_t precision = 8;
};
const std::string xdmf_data<double>::data_type = "Float";

template <typename T>
mesh::MeshTags<T>
XDMFFile::read_meshtags(const mesh::Mesh<double>& mesh, std::string name,
                        std::optional<std::string> attribute_name,
                        std::string xpath)
{
  // omitted stuff...
  values_data_node = attribute_node.child("DataItem");

  const std::string data_type = values_data_node.attribute("DataType").value();
  const auto precision
      = std::stol(values_data_node.attribute("Precision").value());

  if (xdmf_data<T>::data_type != data_type
      || xdmf_data<T>::precision != precision)
  {
    throw std::runtime_error(
        "The data in the XDMF file does not match the required data type.");
  }

  // continue...

What do you think of this approach? Needs polishing obviously... @garth-wells @jhale