fixed time origin on o1 for RSF writier and made dummy starttime vari…

…able

dascore/io/rsf/core.py

@@ -36,6 +36,9 @@ def write(self, spool, path, data_path=None, **kwargs):
 
         spool needs to have a single patch in it
 
+        time origin is forced to 0.0 in the new RSF file, but a dummy
+        variable named 'starttime' still holds the real start time
+
         Parameters
         ----------
         spool
@@ -79,7 +82,11 @@ def write(self, spool, path, data_path=None, **kwargs):
         hdr_info = [hdr_str, file_formt, f"esize={file_esize}"]
         for i in range(length):
             hdr_info.append(f"n{i+1}={axis_lengs[i]}")
-            hdr_info.append(f"o{i+1}={axis_origs[i]}")
+            if i == 0:
+                hdr_info.append(f"o{i+1}=0.0")
+                hdr_info.append(f"starttime={axis_origs[i]}")
+            else:
+                hdr_info.append(f"o{i+1}={axis_origs[i]}")
             hdr_info.append(f"d{i+1}={axis_steps[i]}")
             hdr_info.append(f'label{i+1}="{axis_names[i]}"')
             hdr_info.append(f'unit{i+1}="{axis_units[i]}"')