(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/usercode.py

@@ -0,0 +1,68 @@
+"""
+usercode.py isn't a real module, but an example of a module produced by gencode.py from the
+user-defined document schema.
+
+It is the same code that's produced from the test schema in test_gencode.py. In fact, it is used
+as part of that test.
+
+User-defined Tables (i.e. classes that derive from grist.Table) automatically get some additional
+members:
+
+  Record - a class derived from grist.Record, with a property for each table column.
+  RecordSet - a class derived from grist.Record, with a property for each table column.
+  RecordSet.Record - a reference to the Record class above
+
+======================================================================
+import grist
+from functions import *       # global uppercase functions
+import datetime, math, re     # modules commonly needed in formulas
+
+
+@grist.UserTable
+class Students:
+  firstName = grist.Text()
+  lastName = grist.Text()
+  school = grist.Reference('Schools')
+
+  def fullName(rec, table):
+    return rec.firstName + ' ' + rec.lastName
+
+  def fullNameLen(rec, table):
+    return len(rec.fullName)
+
+  def schoolShort(rec, table):
+    return rec.school.name.split(' ')[0]
+
+  def schoolRegion(rec, table):
+    addr = rec.school.address
+    return addr.state if addr.country == 'US' else addr.region
+
+  @grist.formulaType(grist.Reference('Schools'))
+  def school2(rec, table):
+    return Schools.lookupFirst(name=rec.school.name)
+
+
+@grist.UserTable
+class Schools:
+  name = grist.Text()
+  address = grist.Reference('Address')
+
+
+@grist.UserTable
+class Address:
+  city = grist.Text()
+  state = grist.Text()
+
+  def _default_country(rec, table):
+    return 'US'
+  country = grist.Text()
+
+  def region(rec, table):
+    return {'US': 'North America', 'UK': 'Europe'}.get(rec.country, 'N/A')
+
+  def badSyntax(rec, table):
+    # for a in b
+    # 10
+    raise SyntaxError('invalid syntax on line 1 col 11')
+======================================================================
+"""