Draft: Enhancement/improve ecmwf crawler

• The ECMWF data crawler now downloads a fallback variable in case the desired ECMWF ERA dataset does not cover the requested time interval. This allows to download ECMWF data for dates before 6/2012.
• Added some bug fixes, comments and improved code style in the ECMWF module.
• Added additional ERA interim setups for T2M, O3, and SLP.
• Reordered ERA setups.
• Unavailable ECMWF variables are now automatically skipped.
• Added ECMWF crawler tests.

Fixes issue #75.