File locking

Create a temp file for file locking
This isn't going to work, for two reasons:

1. If the lock file doesn't exist, this will just fail - you need to open it for writing to solve this (`open(...,'w')`).
2. The `tmp` directory doesn't exist in the current directory. You can solve this in one of two ways: 1) use an absolute path, so `'/tmp/massDOT.lock'`, rather than `os.getcwd()+...` or 2) use `os.getcwd() + 'massDOT.lock'`. Either solution seems sensible to me.

Please test this by actually running it before coming back to me.