Change the default number of interpolate_shells for the formal integr…

…al (#1512)

tardis/io/schemas/spectrum.yml

@@ -28,9 +28,11 @@ properties:
             the calculation of the integrated spectrum
       interpolate_shells:
         type: number
-        default: -1
+        default: 0
         description: Number of shells on which the formal
-            integral quantities are interpolated
+            integral quantities are interpolated. For -1 no interpolation
+            is used. The default is to use twice the number of computational
+            shells but at least 80.
   virtual:
     type: object
     default: {}

tardis/montecarlo/formal_integral.py

@@ -84,13 +84,20 @@ def raise_or_return(message):
         return True
 
     def calculate_spectrum(
-        self, frequency, points=None, interpolate_shells=-1, raises=True
+        self, frequency, points=None, interpolate_shells=0, raises=True
     ):
         # Very crude implementation
         # The c extension needs bin centers (or something similar)
         # while TARDISSpectrum needs bin edges
         self.check(raises)
         N = points or self.points
+        if interpolate_shells == 0:  # Default value
+            interpolate_shells = max(2 * self.model.no_of_shells, 80)
+            warnings.warn(
+                "The number of interpolate_shells was not "
+                f"specified. The value was set to {interpolate_shells}."
+            )
+
         self.interpolate_shells = interpolate_shells
         frequency = frequency.to("Hz", u.spectral())
         luminosity = u.Quantity(formal_integral(self, frequency, N), "erg") * (