message: Use `static_cast` to silence warnings in Mine
  Ideally the XML serialization should be using an unsigned type, however
  it doesn't appear to support serializing unsigned values.
diff: @@ -343,7 +343,7 @@ void Mine::deserialize(NAS2D::Xml::XmlElement* element)
  this->active(active != 0);
  mProductionRate = static_cast<MineProductionRate>(yield);
  - mVeins.resize(depth);
  + mVeins.resize(static_cast<std::size_t>(depth));
  for (XmlNode* vein = element->firstChild(); vein != nullptr; vein = vein->nextSibling())
  {
  auto _mv = MineVein{0, 0, 0, 0};
  @@ -358,6 +358,6 @@ void Mine::deserialize(NAS2D::Xml::XmlElement* element)
  else if (attribute->name() == "id") { attribute->queryIntValue(id); }
  attribute = attribute->next();
  }
  - mVeins[id] = _mv;
  + mVeins[static_cast<std::size_t>(id)] = _mv;
  }
  }
  