Set t_min to first day of month for monthly resolution

Adjust t_min to the first day of the month based on schedule start date.

Author: j-atkins

src/virtualship/instruments/base.py

@@ -245,7 +245,9 @@ def _find_files_in_timerange(
             )
 
         if t_resolution == "monthly":
-            t_min = schedule_start.date()
+            t_min = schedule_start.date().replace(
+                day=1
+            )  # first day of month of the schedule start date
             t_max = (
                 schedule_end.date()
                 + timedelta(