Added required AUTHORS and NOTICE files.

Author: Laurent Gougeon

AUTHORS

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+OpenECPDS authors
+=================
+
+The main development of OpenECPDS is done at the European Centre for Medium-Range Weather Forecasts (ECMWF - http://www.ecmwf.int)
+
+Current developers
+==================
+Laurent Gougeon
+Cristina-Iulia Bucur
+
+Past developers
+===============
+Daniel Varela Santoalla
+
+Thanks for contributions from:
+==============================
+
+List of software packages which are contributed inside OpenECPDS:
+=================================================================
+
+- S3Proxy (https://github.com/gaul/s3proxy)
+    Copyright (C) 2014-2021 Andrew Gaul
+    Apache License 2.0 (http://www.apache.org/licenses/)

NOTICE

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+OpenECPDS
+=========
+
+Copyright 2004- ECMWF
+
+This product is developed by the
+
+  Development Section, European Centre for Medium-Range Weather Forecasts (ECMWF) - http://www.ecmwf.int
+
+Below is a list of software packages which are used inside OpenECPDS:
+
+- S3Proxy (https://github.com/gaul/s3proxy)
+    Copyright (C) 2014-2021 Andrew Gaul
+    Apache License 2.0 (http://www.apache.org/licenses/)