Only use omerosql param if it's explicitly passed. Use a tempfile for downloads.

Fix a couple of inconveniences:

- If a new database is being initialised and the default value of the `--omerosql` parameter exists the file named is used instead of being overwritten. This has been changed so it is only read and loaded if `--omerosql` is explicitly set.
- omego checks for the existence of a downloaded file and does not re-download if it exists. This means a partial file from an interrupted download (e.g. network disconnection, or Ctrl-C) would never be replaced. The new behaviour is to download to a tempfile and rename it on success.