Rename LinearReaction and PadeApproximant classes.
Linear reactions and PadeApproximant cleanup.

- rename class members
- reaction matrix - replace 2d array with vectors
- clean up code
Cleanup PadeApproximant.

- remove dead code
- unify the methods with LinearReaction
- unify reaction matrix multiplication
Reactions cleanup, include cleanup.
Reuse output vector scatter in DualPorosity and SorptionBase.
DualPorosity computation scheme switching.
Clean and rename variables in DualPorosity-compute_reaction.
Introduce record scheme_tolerance in DualPorosity.
Reference manual - describe DualPorosity scheme tolerance condition.
Merge with 1.8.0_master.
Merge branch 'PE_reactions_cleanup' into 1.8.0_master