Hips: try to use caching as much as possible

We don't want to send too many requests to the hips servers.  There's
probably more to do in order to make sure we don't make too much access.

src/core/StelHips.cpp

@@ -74,6 +74,7 @@ HipsSurvey::HipsSurvey(const QString& url_, double releaseDate_):
 {
 	// Immediatly download the properties.
 	QNetworkRequest req = QNetworkRequest(getUrlFor("properties"));
+	req.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache);
 	QNetworkReply* networkReply = StelApp::getInstance().getNetworkAccessManager()->get(req);
 	connect(networkReply, &QNetworkReply::finished, [&, networkReply] {
 		QByteArray data = networkReply->readAll();
@@ -152,6 +153,7 @@ bool HipsSurvey::getAllsky()
 		QUrl path = getUrlFor(QString("Norder%1/Allsky.%2").arg(getPropertyInt("hips_order_min", 3)).arg(ext));
 		qDebug() << "Load allsky" << path;
 		QNetworkRequest req = QNetworkRequest(path);
+		req.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache);
 		networkReply = StelApp::getInstance().getNetworkAccessManager()->get(req);
 		emit statusChanged();