(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_treeview.py

@@ -0,0 +1,59 @@
+from collections import namedtuple
+import unittest
+
+from treeview import fix_indents
+
+Item = namedtuple('Item', 'id indentation')
+
+def fix_and_check(items, changes):
+  #  convert from strings to items with ids and indents (e.g. "A0" -> {id: "A", indent: 0} returns
+  #  the pair (adjustments, resulting items converted to strings) for verification
+  all_items = [Item(i[0], int(i[1:])) for i in items]
+  adjustments = fix_indents(all_items, changes)
+  fix_map = {id: indentation for id, indentation in adjustments}
+  all_items = [i for i in all_items if i.id not in changes]
+  result = ['%s%s' % (i.id, fix_map.get(i.id, i.indentation)) for i in all_items]
+  return (adjustments, result)
+
+class TestTreeView(unittest.TestCase):
+
+  def test_fix_indents(self):
+    self.assertEqual(fix_and_check(["A0", "B0", "C1", "D1"], {"B"}), (
+      [("C", 0)],
+      ["A0", "C0", "D1"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C1", "D1"], {"B"}), (
+      [],
+      ["A0", "C1", "D1"]))
+
+    self.assertEqual(fix_and_check(["A0", "B0", "C1", "D2", "E3", "F2", "G1", "H0"], {"B"}), (
+      [("C", 0), ("D", 1), ("E", 2)],
+      ["A0", "C0", "D1", "E2", "F2", "G1", "H0"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C1", "D1"], {"A", "B"}), (
+      [("C", 0)],
+      ["C0", "D1"]))
+
+    self.assertEqual(fix_and_check(["A0", "B0", "C1", "D1"], {"A", "B"}), (
+      [("C", 0)],
+      ["C0", "D1"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C2", "D0"], {"A", "B"}), (
+      [("C", 0)],
+      ["C0", "D0"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C2", "D0"], {"A", "C"}), (
+      [("B", 0)],
+      ["B0", "D0"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C2", "D0"], {"B", "C"}), (
+      [],
+      ["A0", "D0"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C2", "D0", "E0"], {"B", "D"}), (
+      [("C", 1)],
+      ["A0", "C1", "E0"]))
+
+    self.assertEqual(fix_and_check(["A0", "B1", "C2", "D0", "E1"], {"B", "D"}), (
+      [("C", 1), ("E", 0)],
+      ["A0", "C1", "E0"]))