Change default license from "CC BY 4.0" to "CC0"

* Written in the first document of the submission.yaml file, although
  it is not supported by the JSON schema or the HEPData web application.

examples/Getting_started.ipynb

@@ -406,6 +406,11 @@
       "  from theoretical calculations in the bulk graviton and heavy vector triplet models,\n",
       "  spin-2 WW resonances with mass smaller than 1.07~TeV and spin-1 WZ resonances lighter\n",
       "  than 3.05~TeV, respectively, are excluded at 95\\% confidence level.\n",
+      "data_license:\n",
+      "  description: CC0 enables reusers to distribute, remix, adapt, and build upon the\n",
+      "    material in any medium or format, with no conditions.\n",
+      "  name: CC0\n",
+      "  url: https://creativecommons.org/publicdomain/zero/1.0/\n",
       "record_ids:\n",
       "- id: 1657397\n",
       "  type: inspire\n",

hepdata_lib/__init__.py

@@ -569,11 +569,11 @@ def __init__(self):
     def get_license():
         """Return the default license."""
         data_license = {}
-        data_license["name"] = "cc-by-4.0"
-        data_license["url"] = "https://creativecommons.org/licenses/by/4.0/"
-        data_license[
-            "description"] = "The content can be shared and adapted but you must\
-             give appropriate credit and cannot restrict access to others."
+        data_license["name"] = "CC0"
+        data_license["url"] = "https://creativecommons.org/publicdomain/zero/1.0/"
+        data_license["description"] = (
+            "CC0 enables reusers to distribute, remix, adapt, and build upon the material "
+            "in any medium or format, with no conditions.")
         return data_license
 
     def add_table(self, table):