Added references.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

pyrsr/data/Landsat-4/reference

+URL: https://landsat.usgs.gov/landsat/spectral_viewer/bands/L4_TM_RSR.xlsx
+DATE: 08/2016 (?)
+AUTHORSHIP: NASA

pyrsr/data/Landsat-5/reference

+URL: https://landsat.usgs.gov/landsat/spectral_viewer/bands/L5_TM_RSR.xlsx
+DATE: 11/2013 (?)
+AUTHORSHIP: NASA

pyrsr/data/Landsat-7/reference

+URL: https://landsat.usgs.gov/landsat/spectral_viewer/bands/L7_RSR.xlsx
+DATE: 11/2013 (?)
+AUTHORSHIP: NASA

pyrsr/data/Landsat-8/reference

+URL: https://landsat.gsfc.nasa.gov/wp-content/uploads/2014/09/Ball_BA_RSR.v1.2.xlsx
+DATE: 09/2014
+AUTHORSHIP: NASA

pyrsr/data/RapidEye/reference

+URL: https://resa.blackbridge.com/files/2014-06/Spectral_Response_Curves.pdf
+DATE: 06/2014
+AUTHORSHIP: BlackBridge

pyrsr/data/Terra/reference

+URL: https://landsat.usgs.gov/landsat/spectral_viewer/bands/ASTER-RSR.xlsx
+DATE: 12/2017
+AUTHORSHIP: NASA