Use analysis nbsearch in insert-molecules

Advantages:
 - This reduces the amount of code by ~90% compared to what addconf.c
   has, making it significantly easier to understand.
 - Now the tool is independent of potential changes in the
   mdrun-specific neighborhood search.
 - Memory leaks related to addconf.c are gone.
 - The neighborhood search is terminated as soon as one pair within the
   cutoff is found, potentially making it faster. This likely offsets
   any performance differences between the nbsearch implementations.
   The unit tests are ~35% faster.
 - Confusing mdrun-specific output related to the neighborhood
   searching is gone. This includes notes that "This file uses the
   deprecated 'group' cutoff_scheme" and references to Coulomb or VdW
   tables and cutoffs.

Change-Id: Iba82858b9a2b43b6e10a49cd3964b99b22996166