JS mass balance  - second 

Yet need to deal with changes in deleted files:
linear_reaction.cc
linear_reaction_base.cc
- Why here is csection if it does not appear in dual porosity calculation?
- With this approach it will be nightmare to keep consistency with other calculations more over it seems that this is actually more work then dual porosity itself. I propose to just compute deference made by whole reaction term
and possibly only in mobile zone. Changes in immobile zone usually can not be compared to any measurements. 

Anyway, does current approach allow also  reduced "cheap" balance in explicit solver?

Balance servers to two purposes: 1) provide some cumulative data to the user
2) indicate possible error in the calculation
The first one could be partly computed from solution and partly depends on more detailed bookkeeping.
The second have to perform some redundant calculations that should take just fraction of the whole time or we can turn them off.