10 base multi adaptor clean

[EddyCMWF]

See #10 for more details.

The URL adaptor has been refactored so that:

• The download_format code can be used by other adaptors
• It is possible to return a list of files, i.e. without archiving. This is useful for the multi-adaptor as otherwise it would require unpacking and repacking. It may also be useful for users that want to access the list of files uncompressed

[gbiavati]

The code is fine and we should merge asap.
A comment about logging here is going to be useful. Probably after we should implement logging as a tool and update the package.