Initialize SWE-GLib from each class init function

This is to satisfy #8. Also, this makes SWE-GLib available for GLib
before 2.36, as gswe_init() also calls g_type_init(), which was necessary
before GLib 2.36

src/gswe-enumtypes.c.template

@@ -16,6 +16,7 @@
  * You should have received a copy of the GNU General Public License
  * along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
+#include "swe-glib.h"
 #include "gswe-enumtypes.h"
 #include "@filename@"
 
@@ -31,6 +32,8 @@ GType
 {
     static volatile gsize g_define_type_id__volatile = 0;
 
+    gswe_init();
+
     if (g_once_init_enter(&g_define_type_id__volatile)) {
         static const G@Type@Value values[] = {
 /*** END value-header ***/

src/gswe-timestamp.c

@@ -865,6 +865,8 @@ gswe_timestamp_get_julian_day(GsweTimestamp *timestamp, GError **err)
 GsweTimestamp *
 gswe_timestamp_new(void)
 {
+    gswe_init();
+
     return GSWE_TIMESTAMP(g_object_new(GSWE_TYPE_TIMESTAMP, NULL));
 }
 
@@ -886,7 +888,11 @@ gswe_timestamp_new(void)
 GsweTimestamp *
 gswe_timestamp_new_from_gregorian_full(gint year, gint month, gint day, gint hour, gint minute, gint second, gint microsecond, gdouble time_zone_offset)
 {
-    GsweTimestamp *timestamp = GSWE_TIMESTAMP(g_object_new(GSWE_TYPE_TIMESTAMP,
+    GsweTimestamp *timestamp;
+
+    gswe_init();
+
+    timestamp = GSWE_TIMESTAMP(g_object_new(GSWE_TYPE_TIMESTAMP,
                 "gregorian-year",            year,
                 "gregorian-month",           month,
                 "gregorian-day",             day,