wip: Expose HDF5 type conversion handling

[lukas-weber]

This PR introduces a way to control the currently hard-coded handling of type mismatches between HDF5 files and what is requested on the C++ side,
which will simply print a warning if types are not different.

HDF5 has rich support to customize this behavior via dataset transfer property list and the associated type_conv callback. h5 now maintains a default transfer property list on the library global scope. This transfer property list is passed to every HDF5 read and write call that takes it. Setting a callback is exposed to the user via h5::set_type_conv_cb.

The callback closely mirrors what HDF5 does, up to two additional calling conditions h5::conv_except::not_equal (types not equal) h5::conv_except::narrower (dst type is narrower), for which HDF5 will just handle silently unless the value being read/written will actually not fit in the destination. h5 calls the callback manually for that reason to reproduce the old behavior.

API Example

#include <h5/h5.hpp>

// Custom datatype-conversion callback: only fail when a *stored value* cannot be
// represented in the destination type. Merely differing / narrower types are fine
// as long as every value actually fits.
h5::set_type_conv_cb(
   [](h5::conv_except except, h5::datatype src, h5::datatype dst, void *src_buf, void *dst_buf) -> h5::conv_ret {
     switch (except) {
       // Type-level mismatches detected by h5 (no offending element): tolerate them.
       // These come with src_buf == dst_buf == nullptr.
       case h5::conv_except::not_equal:
       case h5::conv_except::narrower:
         return h5::conv_ret::unhandled;   // let HDF5 perform the conversion

       // HDF5's native per-element exceptions: a concrete value can't be represented
       // exactly in `dst` (out of range, precision/truncation loss, non-finite).
       // src_buf / dst_buf point at the offending element.
       case h5::conv_except::range_hi:
       case h5::conv_except::range_low:
       case h5::conv_except::precision:
       case h5::conv_except::truncate:
       case h5::conv_except::pinf:
       case h5::conv_except::ninf:
       case h5::conv_except::nan:
         std::cerr << "value not representable converting "
                   << h5::get_name_of_h5_type(src) << " -> " << h5::get_name_of_h5_type(dst) << "\n";
         return h5::conv_ret::abort;       // h5 turns this into a thrown exception
     }
     return h5::conv_ret::unhandled;
   });