Create an ETMs table auto-purge

[demiangomez]

Whenever there's a version change, the ETMs table should be auto-purged before running the etms generation script.

[demiangomez]

Partially implemented this using the file date of pyETM.py, but there is a problem: sometimes the file date changes without any actual code changes. Needs to handle actual code differences.

[demiangomez]

Implemented a VERSION field in the module which has to be manually changed every time we introduce a significant modification. Marking as resolved.