import cleanups

• moved saqc.core.register._isflagged to saqc.lib.tool.isflagged
• moved saqc.core.{reader, visitor} to saqc.parsing.{reader, visitor}
• added saqc.core.frame module that acts as a bridge to out data-collection-class (currently dios.DictOfSeries)
• removed all direct imports of dios
• cleand up imports (always import from the highest possible level)
• moved SimpleScheme from saqc.core.basescheme.py to new file saqc.core.translation.simplescheme.py
• cleaned constants.py to only hold real constants

i removed all direct references to dios, to simplify maintenance of out dataclass and in preparation of the new data container