Allow using lists of kwargs in morph_stats features (#1021)

Makes possible to specify a list of kwargs in morph-stats to execute the same feature
multiple times with different parameters.

CHANGELOG.rst

@@ -4,6 +4,7 @@ Changelog
 Version 3.2.0
 -------------
 
+- List of multiple kwargs configurations are now allowed in``neurom.apps.morph_stats``.
 - ``neurom.features.neurite.principal_direction_extents`` directions correspond to extents
   ordered in a descending order.
 - Add features ``neurom.features.morphology.(aspect_ration, circularity, shape_factor)```

doc/source/morph_stats.rst

@@ -58,7 +58,7 @@ Short format (prior version 3.0.0)
 An example config:
 
 .. code-block:: yaml
-    
+
     neurite:
         section_lengths:
             - max
@@ -67,13 +67,13 @@ An example config:
             - total
         section_branch_orders:
             - max
-    
+
     neurite_type:
         - AXON
         - APICAL_DENDRITE
         - BASAL_DENDRITE
         - ALL
-    
+
     neuron:
         soma_radius:
             - mean
@@ -93,7 +93,7 @@ function, e.g.
 * ``raw``: array of raw values
 * ``max``, ``min``, ``mean``, ``median``, ``std``: self-explanatory.
 * ``total``: sum of the raw values
-  
+
 An additional field ``neurite_type`` specifies the neurite types into which the morphometrics
 are to be split. It applies only to ``neurite`` features. A sample output using the above
 configuration:
@@ -198,6 +198,65 @@ So the example config from `Short format (prior version 3.0.0)`_ looks:
                - mean
 
 
+List of features format (starting version 3.2.0)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The kwargs entry was converted into a list to allow running the same feature with different arguments. The ``partition_asymmetry`` feature in the example above can be specified multiple times with different arguments as follows:
+
+.. code-block:: yaml
+
+    neurite:
+      partition_asymmetry:
+        kwargs:
+        - method: petilla
+          variant: length
+        - method: uylings
+          variant: branch-order
+        modes:
+        - max
+        - sum
+
+To allow differentiation between the feature multiples, the keys and values of the kwargs are appended at the end of the feature name:
+
+.. code-block::
+
+    partition_asymmetry__variant:length__method:petilla
+    partition_asymmetry__variant:branch-order__method:uylings
+
+The example config from `Short format (prior version 3.0.0)`_ becomes:
+
+.. code-block:: yaml
+
+    neurite:
+      section_branch_orders:
+        kwargs:
+        - {}
+        modes:
+        - max
+      section_lengths:
+        kwargs:
+        - {}
+        modes:
+        - max
+        - sum
+      section_volumes:
+        kwargs:
+        - {}
+        modes:
+        - sum
+    morphology:
+      soma_radius:
+        kwargs:
+        - {}
+        modes:
+        - mean
+    neurite_type:
+    - AXON
+    - APICAL_DENDRITE
+    - BASAL_DENDRITE
+    - ALL
+
+
 Features
 --------