Add test for precision conversion with decl_hdf5

[Yushan-Wang]

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[jbigot]

Do you feel it would be a good candidate to demonstrate the use of #669 ?

[jmorice91]

Perhaps do something like that in case we have issue with ranges,

std::array<std::array<int, N>, N> expected_int_array;

for( int ii=0; ii<test_array.size(); ++ii) {
       std::transform(test_array[ii].cbegin(), test_array[ii].cend(), expected_int_array[ii].begin(), 
                       [](auto aa){ return static_cast<int>(aa); });
}

// comparison
EXPECT_EQ(expected_int_array, read_int_array);