correction to pair_gauss.cpp and suggestions for pair_gauss_cut.cpp

1) Correction to pair_gauss.cpp?

In line 233 of pair_gauss.cpp, it appears as though we forgot to set
cut[j][i] = cut[i][j];

2) Is this a bug?

   Suggestion / feature:

   It is useful to allow mixing rules for pair style gauss. This only
requires changing a few lines of code. I've attached new versions of
pair_gauss.cpp, pair_gauss_cut.cpp, with these changes, and
pair_gauss.txt documentation which describes the changes.

   Usage scenario:

   I currently use pair_style gauss to study the crowded environment
inside a cell, using repulsive gaussian interactions to represent the
steric interactions between different protein complexes in the cell.
(attached image) There thousands of diffferent types of these protein
complexes in the cell, and I use hundreds of types of particles in
these simulations to represent them. Specifying A and B parameters
for each pair of types would be quite tedious (but it's doable).



pair_gauss.txt (6.36 KB)

pair_gauss.cpp (10.1 KB)

pair_gauss_cut.cpp (11 KB)


thanks for your contribution, andrew.
i'll integrate it, check it, and post is as a pull request.