First commit of branch performance:

Bugs fixed:
- Solved an issue with particles being injected with infinite velocity
  resulting in Inf velocity in some cells of the output files.
- Particles are now equally distributed in cylindrical geometry along
  the radial direction.

New features:
- Particles now have their own weight that is recalculated when the
  particle moves to a new cell. This avoid the reduction of density at
  r = 0.

Cases:
- Added a case of Argon flow around a cylinder to measure performance
  and future improvements.

runs/cylFlow/input.json

@@ -0,0 +1,42 @@
+{
+  "output": {
+    "path": "./runs/cylFlow/",
+	"trigger": 100,
+	"cpuTime": false,
+	"numColl": false
+  },
+  "geometry": {
+    "type":     "2DCyl",
+    "meshType": "gmsh",
+    "meshFile": "mesh.msh"
+  },
+  "species": [
+    {"name": "Argon",  "type": "neutral", "mass": 6.633e-26, "weight": 5.0e6}
+  ],
+  "boundary": [ 
+    {"name": "Injection",       "type": "absorption", "physicalSurface": 1},
+    {"name": "Chamber Walls",   "type": "reflection", "physicalSurface": 2},
+    {"name": "Exterior",        "type": "absorption", "physicalSurface": 3}, 
+    {"name": "Cylinder Walls",  "type": "reflection", "physicalSurface": 4},
+    {"name": "Axis",            "type": "axis",       "physicalSurface": 5}
+  ],
+  "inject": [
+    {"name": "Nozzle", "species": "Argon", "flow": 1.0, "units": "sccm", "v": 300.0, "T": [300.0, 300.0, 300.0], "n": [1, 0, 0], "physicalSurface": 1}
+  ],
+  "reference": {
+	"density": 1.0e19,
+	"mass": 6.633e-26,
+	"temperature": 300.0,
+	"radius": 1.88e-10
+  },
+  "case": {
+    "tau":  6.e-7,
+    "time": 3.e-4
+  },
+  "interactions": {
+    "folderCollisions": "./data/collisions/",
+    "collisions": [
+      {"species_i": "Argon", "species_j": "Argon", "crossSections": ["Ar-Ar_Elastic.dat"]}
+    ]
+  }
+}