Finish changes in Fields.

- implement copy constructors, assignment, and
- polymorfphic copy (not yet tested)
- detailed unit tests field_base_test.cpp
- fix several errors in field implementation
- accomodate changed in  fields interface
Merge branch 'JB_unsteady_darcy' of github.com:flow123d/flow123d into JB_unsteady_darcy

Conflicts:
	src/fields/field_impl.hh