From WMO servers to data validation

From WMO servers to data validation

First batch of the process, that does the following:
  - download stations metadata from WMO, and keep the local cache up to date
  - convert metadata into one of its XML equivalent
  - filter unwanted content and fix known mistakes
  - validate the resulting file

The global structure is in place but final operations aren't there yet: more filtering will have to be done in the future.
An incomplete README file has also been added.