Fix Critical Bug and Introduce Better Logging

• Fix a critical bug that was causing us to skip over the entire computation!

• Introduce Spdlog as a dependency, which is a commonly used logging library. This allows us to use python-like logging, taking us from e.g.:

  Info::Inst()->write(
  std::string("Total number of qvectors to compute reduced from ")
  + boost::lexical_cast<std::string>(Params::Inst()->scattering.qvectors.size())
  + std::string(" to ") + boost::lexical_cast<std::string>(qvectors.size())
  + std::string("."));

  to

  Info::Inst()->write("Total number of qvectors to compute reduced from {} to {}.", Params::Inst()->scattering.qvectors.size(), qvectors.size());