Fixed segmentation fault in Coulomb collisions

When the relative velocity between a charged particle and the background
for Coulomb collisions (W in the code) was low, there was a
segmentation fault. This is fixed now as if the norm of the relative
velocity (normW) in the code is too low, no collision is applied.

src/modules/mesh/moduleMesh.f90

@@ -1016,6 +1016,14 @@ MODULE moduleMesh
 
             W = partTemp%part%v - velocity
             normW = NORM2(W)
+            IF (normW > 1.D-10) THEN
+              !If relative velocity is too low, skip collision and move to next particle
+              partTemp => partTemp%next
+
+              CYCLE
+
+            END IF
+              
             lW = l * normW
             AW = coulombMatrix(k)%A_i/normW
 
@@ -1082,6 +1090,14 @@ MODULE moduleMesh
   
               W = partTemp%part%v - velocity
               normW = NORM2(W)
+              IF (normW > 1.D-10) THEN
+                !If relative velocity is too low, skip collision and move to next particle
+                partTemp => partTemp%next
+
+                CYCLE
+
+              END IF
+              
               lW = l * normW
               AW = coulombMatrix(k)%A_j/normW
   

src/modules/solver/moduleSolver.f90

@@ -70,7 +70,6 @@ MODULE moduleSolver
       CHARACTER(:), ALLOCATABLE:: pusherType
       REAL(8):: tau, tauSp
 
-      !TODO: Reorganize if Cart pushers are combined
       SELECT CASE(mesh%dimen)
       CASE(0)
         self%pushParticle => push0D