Correction to injection velocity

Small correction to injection velocity. Still not fully satisfied with
it. I have to find a way to ensure that velocity is injected in the
right direction and fulfills the distribution functions in each
direction regardless the direction of injection, mean velocity or
distribution function.

src/modules/common/moduleRandom.f90

@@ -66,6 +66,24 @@ MODULE moduleRandom
 
     END FUNCTION randomMaxwellian
 
+    FUNCTION randomHalfMaxwellian() RESULT(rnd)
+      USE moduleConstParam, ONLY: PI
+      IMPLICIT NONE
+
+      REAL(8):: rnd
+      REAL(8):: x, y
+
+      rnd = 0.D0
+      x = 0.D0
+      DO WHILE (x == 0.D0)
+        CALL RANDOM_NUMBER(x)
+      END DO
+      y = random(-PI, PI)
+
+      rnd = DSQRT(-2.D0*DLOG(x))*DCOS(y)
+
+    END FUNCTION randomHalfMaxwellian
+
     !Returns a random number weighted with the cumWeight array
     FUNCTION randomWeighted(cumWeight, sumWeight) RESULT(rnd)
       IMPLICIT NONE