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/init/moduleInput.f90

@@ -637,6 +637,7 @@ MODULE moduleInput
       CHARACTER(:), ALLOCATABLE:: electron
       INTEGER:: e
       CLASS(meshCell), POINTER:: cell
+      INTEGER:: subSteps
 
       !Firstly, check if the object 'interactions' exists
       CALL config%info('interactions', found)
@@ -770,8 +771,13 @@ MODULE moduleInput
               pt_i = speciesName2Index(species_i)
               CALL config%get(object // '.species_j', species_j, found)
               pt_j = speciesName2Index(species_j)
+              CALL config%get(object // '.subSteps', subSteps, found)
+              IF (.NOT. found) THEN
+                subSteps = 1
 
-              CALL coulombMatrix(i)%init(pt_i, pt_j)
+              END IF
+
+              CALL coulombMatrix(i)%init(pt_i, pt_j, subSteps)
 
             END DO