Changes file cannot be overwritten
Changes file cannot be overwritten

The example script for _Continuously updating a database_ was fixed to delete the `newchange.osc.gz` file, if exists, before executing `pyosmium-get-changes`.

Otherwise, at the second iteration of the script  `pyosmium-get-changes` would issue an error:
```
Traceback (most recent call last):
  File "/usr/bin/pyosmium-get-changes", line 243, in <module>
    exit(main(sys.argv[1:]))
  File "/usr/bin/pyosmium-get-changes", line 224, in main
    outhandler = WriteHandler(options.outfile)
RuntimeError: Open failed for 'newchange.osc.gz': File exists
```
osmcode/pyosmium,osmcode/pyosmium,osmcode/pyosmium