Add python style of subarray I/O to example programs

[wkliao]

For writing attributes,

    # python subarray style
    var.float_att_name = float_att

    # Equivalently, below uses function call
    var.put_att("float_att_name", float_att)

For variables,

    # python subarray style
    end = [start[i] + count[i] for i in range(2)]
    var[start[0]:end[0], start[1]:end[1]] = buf

    # Equivalently, below uses function call
    var.put_var_all(buf, start = start, count = count)

In addition, move example programs of nonblocking APIs to folder examples/nonblocking