Change default doubling_time from int to float

Author: Eric Smyth

src/penn_chime/settings.py

@@ -18,7 +18,7 @@
         philly=philly,
     ),
     current_hospitalized=14,
-    doubling_time=4,
+    doubling_time=4.0,
     known_infected=510,
     n_days=60,
     market_share=0.15,