revamp EpwDataPoint to use double attributes where appropriate

[jmarrec]

There's a lot of back and forth conversions unnecessarily happening because the attributes are std::string but it should be double (or boost::optional<double> rather). We should limit the number of times we do std::stod conversions. Relates #3700 .

eg

hpp

  std::string m_extraterrestrialDirectNormalRadiation; //units Wh/m2, missing 9999., minimum 0

cpp

  boost::optional<double> EpwDataPoint::extraterrestrialDirectNormalRadiation() const
  {
    if(m_extraterrestrialDirectNormalRadiation == "9999") {
      return boost::none;
    }
    return boost::optional<double>(std::stod(m_extraterrestrialDirectNormalRadiation));
  }

  bool EpwDataPoint::setExtraterrestrialDirectNormalRadiation(double value)
  {
    if(0 > value || value == 9999) {
      m_extraterrestrialDirectNormalRadiation = "9999";
      return false;
    }
    m_extraterrestrialDirectNormalRadiation = std::to_string(value);
    return true;
  }

[jasondegraw]

While there's always room for improvement, using strings for storage was chosen to avoid the (inevitable) loss of precision inherent in string-to-double-to-string conversions, which is why this part of the EPW was added in the first place. Undoing that would be a step backward.