Added the possibility to have sub-steps

Now per each Coulomb collision process there is the possibility to do
sub-steps. This helps in improving accuracy without reducing the time
step of the problem.

src/modules/moduleCoulomb.f90

@@ -12,6 +12,8 @@ MODULE moduleCoulomb
     REAL(8):: lnCoulomb !This can be done a function in the future
     REAL(8):: A_i
     REAL(8):: l2_j
+    REAL(8):: tauSub
+    INTEGER:: nSubSteps
     CONTAINS
       PROCEDURE, PASS:: init => initInteractionCoulomb
 
@@ -44,7 +46,7 @@ MODULE moduleCoulomb
 
     END FUNCTION H
 
-    SUBROUTINE initInteractionCoulomb(self, i, j)
+    SUBROUTINE initInteractionCoulomb(self, i, j, subSteps)
       USE moduleSpecies
       USE moduleErrors
       USE moduleConstParam
@@ -52,10 +54,14 @@ MODULE moduleCoulomb
       IMPLICIT NONE
 
       CLASS(interactionsCoulomb), INTENT(out):: self
+      INTEGER, INTENT(in):: subSteps
       INTEGER, INTENT(in):: i, j
       REAL(8):: Z_i, Z_j
       REAL(8):: scaleFactor
 
+      self%nSubSteps = subSteps
+      self%tauSub    = tauMin / REAL(self%nSubSteps)
+
       self%sp_i => species(i)%obj
       self%sp_j => species(j)%obj