(core) Allow using MIN and MAX functions with dates/datetimes

Summary:
The MIN and MAX functions for formulas previously only considered numbers, ignoring other types, including dates. An example of this being a problem is here: https://community.getgrist.com/t/last-field-circularreferror-what-is-it/1114/4 . Using `MIN` on a column of dates would return 0 (the default) which gets converted to 1970-01-01. Users have to use `min` instead, which is confusing, and doesn't work when some values are empty.

This diff lets the functions operate on date and datetime values. A mixture of dates and datetimes is allowed, even though these cannot usually be compared in Python. Mixing dates and numbers will raise an exception.

Test Plan: Extended doctests

Reviewers: jarek, paulfitz

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3560

sandbox/grist/functions/math.py

@@ -1,6 +1,8 @@
 # pylint: disable=unused-argument
 
 from __future__ import absolute_import
+
+import datetime
 import math as _math
 import operator
 import os
@@ -38,6 +40,13 @@ def _chain_numeric_a(*values_or_iterables):
     yield int(v) if ISLOGICAL(v) else v if ISNUMBER(v) else 0
 
 
+# Iterates through iterable or other arguments, only including numbers, dates, and datetimes.
+def _chain_numeric_or_date(*values_or_iterables):
+  for v in _chain(*values_or_iterables):
+    if ISNUMBER(v) and not ISLOGICAL(v) or isinstance(v, (datetime.date, datetime.datetime)):
+      yield v
+
+
 def _round_toward_zero(value):
   return _math.floor(value) if value >= 0 else _math.ceil(value)