Dual porosity model can be now followed by linear reaction and pade approximant.

- new tests added in test23 but WITHOUT reference data
Fixed bug in set_time - wrong time status after multiple set_time call.
Asserts in set_time display name of field.
Sorption model can follow the dual porosity model from now on.

- dual porosity classes simplified
- new con file in test 23 but without reference data
- sorption model can be constructed in dual porosity from input file data