Modified lb_binary.py to generalize SC model

1. added force couplings between all components
2. added arguments for interaction strengths G11, G12 and G22
3. setting interaction strength G12 sets G21=G12 by default
Modified shan_chen.mako to generalize SC model

1. initialized 'sca0' before function call to shan_chen_force() instead of as 'force' in shan_chen_force()
2. shan_chen_force() is now called for every interaction in the 'force_couplings' dictionary between component 'grid_idx' and component 'dists[1]' unless the 'coupling_const' is equal to 0.0. The results are added to sca0. For example, setting G11 and G12 results in the code:

// for grid_idx = 0:
float sca0[2]={}:
if (isWetNode(type)) {
      shan_chen_force(gi, g0m0, gg0m0, G11, sca0, gx, gy);
      shan_chen_force(gi, g0m0, gg1m0, G12, sca0, gx, gy);
      // Convert momentum and force into velocity and acceleration.
      sca0[0] /= g1m0; sca0[1] /= g1m0;	}

// for grid_idx = 1:
float sca0[2]={}:
if (isWetNode(type)) {
      shan_chen_force(gi, g1m0, gg0m0, G21, sca0, gx, gy);
      // Convert momentum and force into velocity and acceleration.
      sca0[0] /= g1m0; sca0[1] /= g1m0;	}
Created sc_droplet_2d.py

Converted sc_droplet.py to a binary case with self-interaction of component 1.
Changed 'G' to 'G12'
fixed missing endif statement
Fixed issue when G11 and G12 are set

In case both G11 and G12 are set, the flow should now be:
1. Initialize 'sca0'
2. In 'shan_chen_force', initialize and calculate 'force' for G11, add to 'sca0'
3. In 'shan_chen_force', initialize and calculate 'force' for G12, add to 'sca0'
4. 'sca0' is now the combined force G11+G12