Update release number (ECP-WarpX#875)

* update release number

* add message to get list of PR merged since last release

* keep set -e

* Update Tools/update_release.sh

suggested by @ax3l

Docs/source/conf.py

@@ -66,7 +66,7 @@
 # built documents.
 #
 # The short X.Y version.
-version = '20.03'
+version = '20.04'
 # The full version, including alpha/beta/rc tags.
 release = ''
 

LICENSE.txt

@@ -1,4 +1,4 @@
-WarpX v20.03 Copyright (c) 2018, The Regents of the University of California, through Lawrence Berkeley National Laboratory, and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory (subject to receipt of any required approvals from the U.S. Dept. of Energy). All rights reserved.
+WarpX v20.04 Copyright (c) 2018, The Regents of the University of California, through Lawrence Berkeley National Laboratory, and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory (subject to receipt of any required approvals from the U.S. Dept. of Energy). All rights reserved.
 
 
 Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Python/setup.py

@@ -28,7 +28,7 @@
     package_data = {}
 
 setup (name = 'pywarpx',
-       version = '20.03',
+       version = '20.04',
        packages = ['pywarpx'],
        package_dir = {'pywarpx':'pywarpx'},
        description = """Wrapper of WarpX""",

Tools/update_release.sh

@@ -56,4 +56,10 @@ echo ""
 echo "WARNING: Remaining occurences of $old_release_number are listed below."
 echo "         Is this expected? Or should these be updated too?"
 echo ""
-git grep "$old_release_number" .
+git grep "$old_release_number" . || echo ""
+echo "In order to get a list of PRs merged since date <date>, you can run:"
+echo "git log --since=<date> | grep -A 3 \"Author: \" | grep -B 1 \"\-\-\" | sed '/--/d' | sed -e 's/^    /- /'"
+echo "where <date> is replaced by the date since last relate, say 2020-02-02"
+# The actual command is below (commented), the one in echo above
+# has escape characters for printing purpose.
+# git log --since=<date> | grep -A 3 "Author: " | grep -B 1 "\-\-" | sed '/--/d' | sed -e 's/^    /- /'