FieldSet setters affect fields added after.

- FieldSet methods: set_mesh, set_intput_list, set_limit_side
  store values and set them to the fields added after call of the setter

- input/accessors.hh, need is_empty() method for Array; should be in
  common parent class

- fix possible double delete of data in FieldElementwise
Fix test 22
- add fields for new output
- fix ref_output, previous version
  did wrong output of velocity and divergence error
  (computation of divergence error is still wrong,
   however we output right array)
Merge branch 'output_master' of github.com:flow123d/flow123d into output_master