fractional atomic number added for air

src/scgenerator/data/materials.toml

@@ -34,6 +34,7 @@
 [air]
     a             = 0.1358
     atomic_mass   = 4.809e-26
+    atomic_number = 7.14      # 78% N2 and 21 O2
     b             = 3.64e-5
 
 [air.sellmeier]
@@ -52,6 +53,7 @@
 [nitrogen]
     a             = 0.137
     atomic_mass   = 2.3259e-26
+    atomic_number = 7
     b             = 1.709e-5
 
 [nitrogen.sellmeier]

src/scgenerator/physics/materials.py

@@ -78,7 +78,7 @@ class Sellmeier:
 class Gas:
     name: str
     sellmeier: Sellmeier
-    atomic_number: int
+    atomic_number: int | float
     atomic_mass: float
     _n2: float
     ionization_energy: float | None