New and improved method to calculate collisions per iteration:

In each iteration, number of collisions are calculate as a REAL variable
(collFrac) and stored in each cell. The number of collisions is
calculated as FLOOR(collFrac) and, if it is >1 collisions are computed
as usual. Per each collision calculated, 1.0 is removed from collFrac

runs/Argon_Expansion/CX_case.json

@@ -41,14 +41,13 @@
     "radius": 1.88e-10
   },
   "case": {
-    "tau":  [1.0e-5, 1.0e-6],
+    "tau":  [1.0e-6, 1.0e-6],
     "time": 4.0e-3,
     "pusher": ["2DCylNeutral", "2DCylNeutral"],
     "WeightingScheme": "Volume"
   },
   "interactions": {
     "folderCollisions": "./data/collisions/",
-    "tauCollisions": 1e-4,
     "collisions": [
       {"species_i": "Argon", "species_j": "Argon", 
        "cTypes": [