Implement BUFRSounding.valid_by_timestamp()

lib/xmet/bufr.py

@@ -155,3 +155,24 @@ class BUFRSounding(Sounding):
 
         return BUFRSounding.request_sounding(request,
                                              datatimes[-1].validPeriod)
+
+    @staticmethod
+    def valid_by_timestamp(station: str, timestamp: datetime.datetime) -> Self:
+        epoch     = timestamp.timestamp()
+        request   = BUFRSounding.create_request(station)
+
+        datatimes = DataAccessLayer.getAvailableTimes(request)
+        by_delta  = dict()
+
+        for datatime in datatimes:
+            ts    = datatime.getRefTime().time / 1000.0
+            delta = epoch - ts
+
+            by_delta[delta] = datatime
+
+        for delta in sorted(by_delta.keys()):
+            if delta < 0:
+                continue
+
+            return BUFRSounding.request_sounding(request,
+                                                 by_delta[delta].validPeriod)