load_observation: permit user to request dets=[]

Sometimes you just want to look at the pointing.  This is not terribly
time efficient, since the whole file has to be read regardless.  But
we could imagine superior data formats where the ancillary data is
stored separately from the bolometers.

sotodlib/data/load.py

@@ -264,6 +264,9 @@ def load_file(filename, dets=None, signal_only=False):
     Args:
       filename (str or list): A filename or list of filenames (to be
         loaded in order).
+      dets (list of str): The detector names of interest.  If None,
+        loads all dets present in this file.  To load only the
+        ancillary data, pass an empty list.
       signal_only (bool): If set, then only 'signal' is collected and
         other stuff is ignored.
 
@@ -296,7 +299,10 @@ def load_file(filename, dets=None, signal_only=False):
         dets = list(streams['signal'].keys())
 
     # Create AxisManager now that we know the sample count.
-    count = sum(map(len,streams['signal'][dets[0]]))
+    if len(dets) == 0:
+        count = sum(map(len,streams['timestamps']))
+    else:
+        count = sum(map(len,streams['signal'][dets[0]]))
     aman = core.AxisManager(
         core.LabelAxis('dets', dets),
         core.OffsetAxis('samps', count, 0),
@@ -332,8 +338,9 @@ def load_file(filename, dets=None, signal_only=False):
 
     # Copy in the signal, for each file.
     for det_name, arrs in streams['signal'].items():
-        i = list(aman.dets.vals).index(det_name)
-        hstack_into(aman.signal[i], arrs)
+        if dets is None or det_name in dets:
+            i = list(aman.dets.vals).index(det_name)
+            hstack_into(aman.signal[i], arrs)
 
     del streams
     return aman
@@ -352,7 +359,8 @@ def load_observation(db, obs_id, dets=None, prefix=None):
         set.
       obs_id (str): The identifier of the observation.
       dets (list of str): The detector names of interest.  If None,
-        loads all dets present in this observation.
+        loads all dets present in this observation.  To load
+        only the ancillary data, pass an empty list.
       prefix (str): The root address of the data files.  If not
         specified, the prefix is taken from the ObsFileDB.
 
@@ -388,6 +396,10 @@ def load_observation(db, obs_id, dets=None, prefix=None):
     for p in pairs_req:
         dets_by_detset[p[0]].append(p[1])
 
+    # If user requested no dets, include one detset to get ancil.
+    if len(dets_by_detset) == 0:
+        dets_by_detset[pairs[0][0]] = []
+
     # Loop through relevant files, in sample order, and accumulate
     # lists of stream segments.
     aman = None
@@ -418,7 +430,7 @@ def load_observation(db, obs_id, dets=None, prefix=None):
 
         if aman is None:
             # Create AxisManager now that we know the sample count.
-            count = sum(map(len,streams['signal'][dets[0]]))
+            count = sum(map(len,streams['timestamps']))
             aman = core.AxisManager(
                 core.LabelAxis('dets', [p[1] for p in pairs_req]),
                 core.OffsetAxis('samps', count, 0, obs_id),