First Coulomb implementation that works

After fixing all possible divisions by zero I was able to find in the
Coulomb collision I think that this is a first working implementation of
a Coulomb operator based on moments.

Still to test a few things, modify the manual but I would say that I'm
satisfiyed right now. This operator won't be used that often but maybe
improving efficiency is still needed.

In the future a binary operator is required to be able to study cases
out of Maxwellian equilibrium.

src/modules/mesh/2DCyl/moduleMesh2DCyl.f90

@@ -163,7 +163,7 @@ MODULE moduleMesh2DCyl
       r2 = self%n2%getCoordinates()
       self%z  = (/r1(1), r2(1)/)
       self%r  = (/r1(2), r2(2)/)
-      self%weight = SUM(self%r)*5.D-1
+      self%weight = r2(2)**2 - r1(2)**2
       !Normal vector
       self%normal = (/ -(self%r(2)-self%r(1)), &
                          self%z(2)-self%z(1) , &