(core) move data engine code to core

Summary:
this moves sandbox/grist to core, and adds a requirements.txt
file for reconstructing the content of sandbox/thirdparty.

Test Plan:
existing tests pass.
Tested core functionality manually.  Tested docker build manually.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2563

sandbox/grist/test_functions.py

@@ -0,0 +1,29 @@
+import doctest
+import functions
+import moment
+
+_old_date_get_global_tz = None
+
+def date_setUp(doc_test):
+  # pylint: disable=unused-argument
+  global _old_date_get_global_tz # pylint: disable=global-statement
+  _old_date_get_global_tz = functions.date._get_global_tz
+  functions.date._get_global_tz = lambda: moment.tzinfo('America/New_York')
+
+def date_tearDown(doc_test):
+  # pylint: disable=unused-argument
+  functions.date._get_global_tz = _old_date_get_global_tz
+
+
+# This works with the unittest module to turn all the doctests in the functions' doc-comments into
+# unittest test cases.
+def load_tests(loader, tests, ignore):
+  tests.addTests(doctest.DocTestSuite(functions.date, setUp = date_setUp, tearDown = date_tearDown))
+  tests.addTests(doctest.DocTestSuite(functions.info, setUp = date_setUp, tearDown = date_tearDown))
+  tests.addTests(doctest.DocTestSuite(functions.logical))
+  tests.addTests(doctest.DocTestSuite(functions.math))
+  tests.addTests(doctest.DocTestSuite(functions.stats))
+  tests.addTests(doctest.DocTestSuite(functions.text))
+  tests.addTests(doctest.DocTestSuite(functions.schedule,
+                                      setUp = date_setUp, tearDown = date_tearDown))
+  return tests