message: Use static_cast from double to float in XML serialization
  Prevents clang compiler warning
diff: @@ -438,7 +438,8 @@ void TileMap::serialize(NAS2D::Xml::XmlElement* element, const Planet::Attribute
  properties->attribute("sitemap", planetAttributes.mapImagePath);
  properties->attribute("tset", planetAttributes.tilesetPath);
  properties->attribute("diggingdepth", planetAttributes.maxDepth);
  - properties->attribute("meansolardistance", planetAttributes.meanSolarDistance);
  + // NAS2D only supports double for floating point conversions as of 26July2020
  + properties->attribute("meansolardistance", static_cast<double>(planetAttributes.meanSolarDistance));
  // ==========================================
  // VIEW PARAMETERS
  // ==========================================
  