EquationBase methods empty implementation.

- virtual methods are empty and gives debug message if called unimplemented
- add method initialize()
- remove EquationNothing
- set flag equation_empty_ if default constructor is called
Reactionterm initialization methods reorganizations.

- simplify constructors
- remove set_porosity methods
- introduce initialize() method
- group initialization of data fields in single method
- group intialization from input in single method
Merge branch '1.8.0_master' of https://github.com/flow123d/flow123d into 1.8.0_master