Add initial top-level workflow

This MR creates an initial top-level CWL workflow for the pre-processing pipeline, which calls the preprocessing step: preprocess.cwl. Additionally, a workflow test input JSON is provided.

Note: the output MSs are created by appending .prepout to the original name of the MS. This is temporary and subject to change.

tests/pipeline_input.json

+{
+    "msin": [
+        {
+            "class": "Directory",
+            "path": "../../../data/raw/L888536_SAP000_SB027_uv.MS"
+        },
+        {
+            "class": "Directory",
+            "path": "../../../data/raw/L888536_SAP000_SB028_uv.MS"
+        }
+    ],
+    "demix_sources": [],
+    "demix_timestep": 10,
+    "demix_freqstep": 64,
+    "avg_timestep": 1,
+    "avg_freqstep": 4
+}