Fix links to USGS NEIC PDE in docstrings

Author: claudiodsf

obspy/io/pde/README.txt

@@ -17,4 +17,4 @@ Epicenters (PDE) Bulletin.
 Currently, only the mchedr (machine readable Earthquake Data Report)
 format is supported, and only for file format revision of February 24, 2004.
 
-See: http://earthquake.usgs.gov/research/data/pde.php
+See: https://earthquake.usgs.gov/data/comcat/catalog/us/

obspy/io/pde/__init__.py

@@ -8,7 +8,7 @@
 Currently, only the mchedr (machine readable Earthquake Data Report)
 format is supported.
 
-.. seealso:: http://earthquake.usgs.gov/data/pde.php
+.. seealso:: https://earthquake.usgs.gov/data/comcat/catalog/us/
 
 :copyright:
     The ObsPy Development Team (devs@obspy.org), Claudio Satriano

obspy/io/pde/mchedr.py

@@ -4,7 +4,7 @@
 
 Only supports file format revision of February 24, 2004.
 
-.. seealso:: http://earthquake.usgs.gov/data/pde.php
+.. seealso:: https://earthquake.usgs.gov/data/comcat/catalog/us/
 
 :copyright:
     The ObsPy Development Team (devs@obspy.org), Claudio Satriano