Fixed bug when loading nnc info for amalagamated LGR

1. Fixed a bug with amalgameted LGR + NNC info.
2. Changed nnc_info implementation:
    - Inner loop is based on ecl_file_select_block("NNCHEAD)"
    - nnc_vector is implmented on top of nnc_index_list; which guarantees a sorted and unique list of indices.