Metadata handling

[tracyscanlon]

Metadata handling in the validation framework should be supported from start to finish.

We want to track:

• metadata from the input datasets. The pygeobase.object_base.TS object already has a .metadata attribute which can be extracted by the data manager.
• metadata generated by the validation framework itself. e.g. which step failed why. See also discussion in https://github.com/TUW-GEO/pytesmo/issues/152

We should implement a metadata object which can take the metadata from the input datasets and the result managers which can write the metadata.