Reorganization of mass balance.
- New class MassBalance separated from the transport equation.
- Balance is calculated at each computational time step, not only at output times.
- Units added to text output.
- Updated reference manual with description of calculated quantities.
- Files mass_balance.txt added to reference results of all relevant tests.

TODO:
- Explicit transport still produces significant error in cumulative bilance, especially at times when boundary condition changes.
- Calculation of mass balance may slow down the explicit transport (to be checked). Possibly implement option to switch off mass balance.
- Balance does not work with reactions.
Optimization of Input::Address class
relative references - message how the reference was expanded
Added to commit #2589: Correct mass_balance.txt reference files.
interpolation tables are updated in the case when adsorption model parameters are changed_during_set_time
exchange of code between Classes Sorption and Isotherm, type of adsorption simulation placed into the class Isotherm
Corrections in calculation of mass balance.
- correct treatment of boundary conditions (both implicit and explicit transport)
- incorporation of solute phases (explicit)
- corrected summation in cumulative formulas (both)

Issues remaining in explicit transport:
- mass balance is not correct if sorption_enabled=true
- mass balance is not correct in presence of sources
Corrected mass balance for explicit transport with sources.
The remaining case, when mass balance does not hold, is explicit transport with sorption.
Optimization and correction of Input::Address class