Add metadata for motion parameters (#3245)

Closes #3231.

## Changes proposed in this pull request

- Add metadata, including Units, for motion parameters (6 basic
parameters and FD).
- I included FD because it might be good to log the head radius that is
used to calculate it at some point (e.g., if
nipreps/nibabies#198 moves forward)

## Documentation that should be reviewed

none

---------

Co-authored-by: Chris Markiewicz <[email protected]>

fmriprep/workflows/bold/confounds.py

@@ -413,10 +413,31 @@ def init_bold_confs_wf(
         name='crowncc_metadata_fmt',
     )
 
+    # Combine all confounds metadata
     mrg_conf_metadata = pe.Node(
-        niu.Merge(3), name='merge_confound_metadata', run_without_submitting=True
+        niu.Merge(4), name='merge_confound_metadata', run_without_submitting=True
     )
+    # Tissue mean time series
     mrg_conf_metadata.inputs.in3 = {label: {'Method': 'Mean'} for label in signals_class_labels}
+    # Movement parameters
+    mrg_conf_metadata.inputs.in4 = {
+        'trans_x': {'Description': 'Translation along left-right axis.', 'Units': 'mm'},
+        'trans_y': {'Description': 'Translation along anterior-posterior axis.', 'Units': 'mm'},
+        'trans_z': {'Description': 'Translation along superior-inferior axis.', 'Units': 'mm'},
+        'rot_x': {
+            'Description': 'Rotation about left-right axis. Also known as "pitch".',
+            'Units': 'rad',
+        },
+        'rot_y': {
+            'Description': 'Rotation about anterior-posterior axis. Also known as "roll".',
+            'Units': 'rad',
+        },
+        'rot_z': {
+            'Description': 'Rotation about superior-inferior axis. Also known as "yaw".',
+            'Units': 'rad',
+        },
+        'framewise_displacement': {'Units': 'mm'},
+    }
     mrg_conf_metadata2 = pe.Node(
         DictMerge(), name='merge_confound_metadata2', run_without_submitting=True
     )