add photo of Arpit

Author: viktorpm

_data/people.yml

@@ -32,7 +32,7 @@ senior_research_fellows:
 research_fellows:
   - name: "Arpit Agarwal"
     title: "Research Fellow"
-    photo: null
+    photo: "arpit_agarwal.jpg"
     description: >-
       Arpit's research focuses on statistical learning and memory formation in
       rodents. He develops behavioral paradigms to study how animals learn and