usm_ndarray should provide const versions of member functions

[diptorupd]

Const getter functions of usm_ndarray should be marked const so that the functions may be used on a const usm_ndarray reference.

E.g.,

    int get_flags() const
    {
        PyObject *raw_o = this->ptr();
        PyUSMArrayObject *raw_ar = reinterpret_cast<PyUSMArrayObject *>(raw_o);

        return UsmNDArray_GetFlags(raw_ar);
    }