bugfixes for cleanup_moltemplate.sh and several OPLS water models

1. I fixed an edge-case bug causing "cleanup_moltemplate.sh" to fail without printing out an error message. This would happen whenever the user ran moltemplate.sh on a file not named "system.lt", and then attempted to run "cleanup_moltemplate.sh" afterwards. (This is a situation that "cleanup_moltemplate.sh" does not handle gracefully.) I did not fix this limitation. But now whenever a user does this, it prints out a detailed error message explaining how to get around this limitation.
2. I fixed some serious mistakes in the "spc_oplsaa.lt" and "tip3p_oplsaa.lt" files (for the SPC and TIP3P water models). Thanks to github user feifzhou for pointing them out!