(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/runtests.py

@@ -0,0 +1,33 @@
+"""
+Helper to run Python unittests in the sandbox. They can be run directly as follows:
+
+  ./sandbox/nacl/bin/run -E PYTHONPATH=/thirdparty python -m unittest discover -v -s /grist
+
+This modules makes this a bit easier, and adds support for --xunit option, needed for running
+tests under 'arc unit' and under Jenkins.
+
+  ./sandbox/nacl/bin/run python /grist/runtests.py [--xunit]
+"""
+import os
+import sys
+import unittest
+sys.path.append('/thirdparty')
+
+def main():
+  # Change to the directory of this file (/grist in sandbox), to discover everything under it.
+  os.chdir(os.path.dirname(__file__))
+
+  argv = sys.argv[:]
+  test_runner = None
+  if "--xunit" in argv:
+    import xmlrunner
+    argv.remove("--xunit")
+    test_runner = xmlrunner.XMLTestRunner(stream=sys.stdout)
+
+  if all(arg.startswith("-") for arg in argv[1:]):
+    argv.insert(1, "discover")
+
+  unittest.main(module=None, argv=argv, testRunner=test_runner)
+
+if __name__ == '__main__':
+  main()