Fix system.hh for case without HAVE_PETSC
Replace get_field by operator[].

- overload operator[] (returns reference, and throws for unknown name)
- field() method returns pointer (nullptr if not found)
Distinguish field "name" and "input_name".

- introduce Field<..>::name attribute which identifies
  the field in a FieldSet
- distinguish this from input_name used to declare and read appropriate key
  in the input filed descriptor
- introduce methon input_name() to read the input_name
- rename setters used just for input
- fix unit tests
Merge branch '1.8.0_master' into JB_finish_fieldset
Temporary fix for wrong ASSERT.
Introduce FlagArray.

- bitfield like class that allows setting and testing
  arbitrary subset of bits.
Introduce FieldFlag class.

- this class just defines FlagArray type used by Field classes
  and common Masks to set and test
- in order to make these masks easily accesible from FieldSet, we make
   FieldSet descendant of FieldFlag
- add FieldCommonBase::flags() getter
Replace FieldCommonBase::just_copy() method

- remove the method, remove is_copy member
- replace it by flag declare_input turned off
- introduce mask input_copy to this end
- add constructor of FlagArray from a Mask
Fix missing include.

- add missing conditional include of petsc.hh
Remove compiler warnings.

- remove base warnings generated during compilation without -W flags
Remove compiler warnings.

- remove warnings generated by -Wsign-compare flag.
Remove compiler warnings.

- remove warnings generated by -Wsign-compare flag.
Remove compiler warnings.

-remove warnings generated by -Wunused-function and
-Wmaybe-uninitialized flags
Remove compiler warnings.

- remove unused variables (warning -Wunused-variable)
Remove compiler warnings.

- remove warning -Wparentheses
Merge branch 'master' into DF_warnings
Remove compiler warnings.

- remove warning -Wreorder
Apply field_flags.

- minor change in flags interface, renaming 'set' method to 'add' method
- apply flags in Sorptions and Transport
- try to remove some virtual methods in Advection Models
Refactor fields.

- rename FieldBase<> template to FieldAlgorithmBase
- rename its files to field_algo_base.*
- rename*_impl.* to *.impl.*
FieldSet::set_n_components

- add FieldSet::set_n_components
- rename FieldCommonBase::n_comp to set_n_components
- apply in Sorption,DualPorosity, TransportDG, Transport
- comment out init_data method of Trnasport models