Fix casting of time points with non-system-clock duration with VS (#1748)

* Fix casting of time points with non-system-clock duration on Windows

Add explicit `time_point_cast` to time point with duration of system
clock. Fixes Visual Studio compile error.

* Add test case for custom time points casting
diff --git a/tests/test_chrono.py b/tests/test_chrono.py
index 2b75bd1..f308de9 100644
--- a/tests/test_chrono.py
+++ b/tests/test_chrono.py
@@ -99,3 +99,9 @@
     diff = m.test_chrono_float_diff(43.789012, 1.123456)
     assert diff.seconds == 42
     assert 665556 <= diff.microseconds <= 665557
+
+
+def test_nano_timepoint():
+    time = datetime.datetime.now()
+    time1 = m.test_nano_timepoint(time, datetime.timedelta(seconds=60))
+    assert(time1 == time + datetime.timedelta(seconds=60))