ENH: Add support for the `pairs_nb` directive

[BvB93]

Adds FOX.TOPContainer support for the GROMACS pairs_nb directive, which is used for specifying explicit non-bonded pairs for atoms that are either more than 3 bonds removed or are not indirectly connected by any bonds at all.

This includes:

• A new TOPContainer.pairs_nb attribute
• The TOPContainer.concatenate.pairs_nb() function for manually adding new pairs_nb entries
• The TOPContainer.generate_pairs_nb() function for automatically generating all pairs given a set of atoms and bonds.

[codecov[bot]]

Codecov Report

Merging #299 (c180d34) into master (b348650) will decrease coverage by 0.10%. The diff coverage is 57.89%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
- Coverage   74.65%   74.55%   -0.10%     
==========================================
  Files          71       71              
  Lines        8111     8144      +33     
  Branches     1719     1725       +6     
==========================================
+ Hits         6055     6072      +17     
- Misses       1614     1626      +12     
- Partials      442      446       +4     

Impacted Files	Coverage Δ
FOX/io/_top_concat.py	59.52% <9.09%> (-3.54%)	:arrow_down:
FOX/recipes/top.py	73.91% <50.00%> (-1.09%)	:arrow_down:
FOX/io/_read_top.py	79.94% <82.60%> (-0.06%)	:arrow_down: