(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/treeview.py

@@ -0,0 +1,32 @@
+"""
+Grist supports organizing a list of records as a tree view which allows for grouping records as
+children of some other record.
+
+On the client, the .indentation is used to measure the distance between the left margin of the
+container and where we want the record to be. The variation of .indentation gives the parent-child
+relationship between consecutive records. For instance in ["A0", "B1", "C1"] (where "A0" stands for
+the record {'id': "A", 'indentation': 0}), "B" and "C" are children of "A". In ["A0", "B1", "C2"],
+"C" is a child of "B", which is a child of "A".
+
+The order for the records is typically handled using a field of type "PositionNumber", ie: .pagePos
+in _grist_Pages table.
+
+Because user can remove records that invalidate the tree, the module exposes fix_indents. For
+example if user removes "C" from ["A0", "B1", "C0", "D1"] the resulting table holds ["A0", "B1",
+"D1"] and "D" became child of "A", which is unfortunate because we'd rather have "C" become a
+sibling of "A" instead. Using fix_indents helps with keeping the tree consistent by returning [("D",
+0)] which indicate that the indentation of row "D" needs to be set to 0.
+"""
+
+# Items is an array of items with .id and .indentation properties. Returns a list of (item_id,
+# new_indent) pairs.
+def fix_indents(items, deleted_ids):
+  max_next_indent = 0
+  adjustments = []
+  for item in items:
+    indent = min(max_next_indent, item.indentation)
+    is_deleted = item.id in deleted_ids
+    if indent != item.indentation and not is_deleted:
+      adjustments.append((item.id, indent))
+    max_next_indent = indent if is_deleted else indent + 1
+  return adjustments