What should we expose through the import machinery

We currently import stuff somewhat randomly into the main saqc namespace. As the top level import also form a public interface, we should come up with a toughtful minimal set of symbols to export.

IMO, the following things need to be exported:

• the decorators flagging and processing
• all translators
• the flagging constants UNFLAGGED, GOOD, BAD and DOUBTFUL
• dios.DictOfSeries. We aim for a package import here (import dios), instead of a subpackage import (i.e. from saqc import dios)

Any other ideas @luenensc and @palmb ?