Finalize NNC related work

1. Have added a very small 'class' nnc_vector which contains a int_vector instance and the number of the corresponding lgr.
2. Have added small class nnc_index_list which implements a sorted & unique list of cell indices of cells which have nnc information.
3. Made the dual porosity check stricter; all fracture cells are ignored.
4. Some basic usage documentation.