Draft: Feature/ecsv container output

Some discussion on molecular atmosphere calibration. The calibrator class was splitted to

1. The initial calibration class, but with reduced functionality: now it only contains the production of the contemporary MDP, the reference MDP selection (more of a placeholder for the moment) and the simulation input for tailored MCs.
2. The external_data_request module. This inherits from the CTA component, and it is part of the core functionality since it is not coupled to atmospheric calibration. In this module we request for data (for the moment from RDA, Copernicus & Keeling curve) and we convert them in format directly digestable by our calibrators

Other discussions: modified the grib2 merging, conversion and cleaning functionality. Since we run via workflows, no directory cleanup is needed. Also simplified a bit the naming scheme.

Output: according to the last discussions, no ecsv output is required. However played a bit with MACOBAC (the simplest one) giving the output in astropy tables or containers. Which is most convenient for DB storage?

PS: unittests are failing but I see that's due to throughput calibration.