feat: add JSONProducer

Outputs a representation of the Features extrated by the pipeline

Intent is to write a FE that takes the output so that a pipeline can be
're-run'

Output JSON could also be used with external tools

pipeline/video_producers.py

@@ -1,6 +1,7 @@
 """Classes for producing videos"""
 
 from abc import ABC
+import json
 import logging
 import os
 import subprocess
@@ -139,3 +140,15 @@ class VisualisationProducer(Producer):
         # save the plot
         plt.savefig("/tmp/visualisation.png")
         plt.close()
+
+class JSONProducer(Producer):
+    """Produce JSON output"""
+    def __init__(self, features):
+        if not features:
+            raise ValueError("No features provided")
+        self.features = features
+
+    def produce(self):
+        # FIXME: config option for output path
+        with open("/tmp/features.json", "w") as jsonfile:
+            jsonfile.write(json.dumps(self.features, cls=PipelineJSONEncoder, indent=4))