(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/testsamples.py

@@ -0,0 +1,43 @@
+import testutil
+
+# pylint: disable=line-too-long
+sample_students = testutil.parse_test_sample({
+  "SCHEMA": [
+    [1, "Students", [
+      [1, "firstName",   "Text",        False, "", "", ""],
+      [2, "lastName",    "Text",        False, "", "", ""],
+      [4, "schoolName",  "Text",        False, "", "", ""],
+      [5, "schoolIds",   "Text",        True, "':'.join(str(id) for id in Schools.lookupRecords(name=$schoolName).id)", "", ""],
+      [6, "schoolCities","Text",        True, "':'.join(r.address.city for r in Schools.lookupRecords(name=$schoolName))", "", ""],
+    ]],
+    [2, "Schools", [
+      [10, "name",        "Text",       False, "", "", ""],
+      [12, "address",     "Ref:Address",False, "", "", ""]
+    ]],
+    [3, "Address", [
+      [21, "city",        "Text",       False, "", "", ""],
+    ]]
+  ],
+  "DATA": {
+    "Students": [
+      ["id","firstName","lastName", "schoolName" ],
+      [1,   "Barack",   "Obama",    "Columbia"   ],
+      [2,   "George W", "Bush",     "Yale"       ],
+      [3,   "Bill",     "Clinton",  "Columbia"   ],
+      [4,   "George H", "Bush",     "Yale"       ],
+      [5,   "Ronald",   "Reagan",   "Eureka"     ],
+      [6,   "Gerald",   "Ford",     "Yale"       ]],
+    "Schools": [
+      ["id",  "name",     "address"],
+      [1,     "Columbia", 11],
+      [2,     "Columbia", 12],
+      [3,     "Yale",     13],
+      [4,     "Yale",     14]],
+    "Address": [
+      ["id",  "city"       ],
+      [11,    "New York"   ],
+      [12,    "Colombia"   ],
+      [13,    "New Haven"  ],
+      [14,    "West Haven" ]],
+  }
+})