[hdf5] Incorrect handling of files in read-write mode

[galexv]

The following code produces a run-time error:

#include <alps/hdf5.hpp>
#include <iostream>

int main(int argc, char** argv)
{
    std::string filename("test.h5");
    const std::vector<int> data {1234,5678};

    alps::hdf5::archive ar(filename, "rw"); // Note (1)
    ar["/data"] << data;

    boolalpha(std::cout);
    // std::cout << "Data? " << ar.is_data("/data"); // Note (3)
    // std::cout << "\nScalar? " << ar.is_scalar("/data"); // Note (3)

    std::vector<int> rd_data;
    ar["/data"] >> rd_data; // Note(2)

    std::cout << "\nExpected data: " << data[0] << "," << data[1];
    std::cout << "\nActual data: " << rd_data[0] << "," << rd_data[1];

    return 0;
}

Notes:
(1) In current implementation, "r" and "rw" modes are equivalent ("r" is ignored).
(2) If the file did not exist when it was opened (e.g., on the first run of the program), the run is successful! Otherwise, this call fails throwing alps::hdf5::path_not_found.
(3) If any (or both) of these lines are uncommented, these calls are successful, but line (2) fails, no matter whether the file existed when it was opened.

Funny enough, a similar code that uses a scalar rather than a vector works correctly.