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