function A = gravAccelNSq (X1, M2, X2)
% function A = gravAccel (X)
%
% Find the acceleration due to gravity from one set of point masses to another.
% Gravitational constant not factored in (ie: set to 1).
% O(n^2) algorithm.
%
% parameters:
%   X1 = (set 1) matrix of positions, where row i is the position of particle i.
%   M2 = (set 2) row vector of masses.
%   X2 = (set 2) matrix of positions, where row i is the position of particle i.
%
% returns:
%   A = acceleration set 1 feels due to set 2.
%
% written by Robert Osada

% distance less than err are ignored (collision)
err = 1e-9;

// TODO