(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/grist.py

@@ -0,0 +1,15 @@
+"""
+This file packages together other modules needed for usercode in order to create
+a consistent API accessible with only "import grist".
+"""
+# pylint: disable=unused-import
+
+# These imports are used in processing generated usercode.
+from usertypes import Any, Text, Blob, Int, Bool, Date, DateTime, \
+Numeric, Choice, Id, Attachments, AltText, ifError
+from usertypes import PositionNumber, ManualSortPos, Reference, ReferenceList, formulaType
+from table import UserTable
+from records import Record, RecordSet
+
+DOCS = [(__name__, (Record, RecordSet, UserTable)),
+        ('lookup', (UserTable.lookupOne, UserTable.lookupRecords))]