Adds inline for functions & Bugfix in (de-)serialization

[Onlynagesha]

Changes:

1. Adds inline keyword for functions to prevent link error;
2. Fixes the bug in current (de-)serialization code that sizeof(N_) may mismatch with sizeof(size_t) since N_ is of type index_t;
3. Changes the representation type of duration in nw::util::timer to double for more precision (especially for seconds_timer)

Example of 2. bug in (de-)serialization:

  void serialize(std::ostream& outfile) {
    size_t el_size = sizeof(indices_[0]);
    size_t st_size = indices_.size();

    outfile.write(reinterpret_cast<const char*>(magic_), sizeof(magic_));
    outfile.write(reinterpret_cast<char*>(&N_), sizeof(size_t)); // <------ BUG HERE !

    outfile.write(reinterpret_cast<char*>(&st_size), sizeof(size_t));
    outfile.write(reinterpret_cast<char*>(&el_size), sizeof(size_t));
    outfile.write(reinterpret_cast<char*>(indices_.data()), st_size * el_size);
    to_be_indexed_.serialize(outfile);
  }