joint indexing=False for stills experiments (#2605)

* Set `joint_indexing=Auto` and resolve to `False` for stills, `True` for scans
* Alert the user when `joint_indexing=Auto` is resolved
* Raise an error if there is a mixture of scans and stills.

newsfragments/2605.bugfix

@@ -0,0 +1,3 @@
+``dials.index``: Set joint indexing to be automatically defined by default:
+``True`` for rotation data, ``False`` for still shots. This default can be
+overridden by the user by explicit use of ``joint_indexing=``.

src/dials/command_line/index.py

@@ -10,6 +10,7 @@
 
 import iotbx.phil
 from dxtbx.model.experiment_list import ExperimentList
+from libtbx import Auto
 
 from dials.algorithms.indexing import DialsIndexError, indexer
 from dials.array_family import flex
@@ -69,7 +70,7 @@
       .type = ints(size=2)
       .multiple = True
 
-    joint_indexing = True
+    joint_indexing = Auto
       .type = bool
 
 }
@@ -167,6 +168,18 @@ def index(experiments, reflections, params):
     if params.indexing.image_range:
         reflections = slice_reflections(reflections, params.indexing.image_range)
 
+    if params.indexing.joint_indexing is Auto:
+        if all(e.is_still() for e in experiments):
+            params.indexing.joint_indexing = False
+            logger.info("joint_indexing=False has been set for stills experiments")
+        elif all(not e.is_still() for e in experiments):
+            params.indexing.joint_indexing = True
+            logger.info("joint_indexing=True has been set for scans experiments")
+        else:
+            raise ValueError(
+                "Unable to set joint_indexing automatically for a mixture of stills and scans experiments"
+            )
+
     if len(experiments) == 1 or params.indexing.joint_indexing:
         indexed_experiments, indexed_reflections = _index_experiments(
             experiments,