Correct passing of cross_section field

- introduce virtual EquationBase::zero_time_step()
  initialization before first update_solution() call but after
  setup of field couplings

- EquationBase::data(), EquationBase::eq_data_ pointer
  abstract interface to pass data between equations

- FieldCommonBase::just_copy() ...
  flag that the field should not be read from input, but will be initialized
  by assignment operator or copy_from method
  ... to be continued

- DarcyFlowMH_Steady:
  rename data -> data_
  delete output_object in destructor

- DarcyFlowMHOutput:
  use direct access to data_ member of the friend class DarcyFlowMH_Staedy
  remove unnecessary input keys
  remove commented old code
  delete output_stream in destructor
  use write_time_frame method to output the stream

- output:
  remove static method write_all_data
  add method write_time_frame, for output of one output_stream

- main: remove artifacts from global output streams

- reactions - dual porosity, sorption - add just_copy flag to
  porosities obtained by copy_from

- transport:
  remove all set_cross_section_field methods
  make corss_section proper field, not mere pointer
  move parts of constructors to zero_time_step method