Restructure API

[lars-reimann]

• Rename top level from safe_ds to safeds
• Group classification and regression in an ml module
• Remove plot_ prefix from plotting functions
• Create modules data.tabular and data.tabular.typing
• Move Table, Column, Row to data.tabular.containers (?)
• Move imputers etc. to data.tabular.transformation
• Move data exceptions to data.exceptions (I prefer to have a central place for exceptions)
• Move learning exceptions to ml.exceptions (ditto)
• Replace SupervisedDataset by TaggedTable.
• Make TaggedTable a subclass of Table.

[SmiteDeluxe]

Plotting as methods of column:

1. Still also seperately?
2. Not for heatmap obviously
3. for scatter- and lineplot also either not or taking the second column as argument?

[lars-reimann]

The remaining tasks are now in their own issues.