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/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