Add copy method for Tables

[zzril]

Is your feature request related to a problem?

The Table and TaggedTable classes should have a dedicated private _copy() method.
This method is helpful for tests (e.g. when we want to verify that a method does not change the original table).
Also, a manual copy mechanism is currently implemented in several places. All of these occurrences should be found and be replaced by the new _copy() method.

These methods will not be intended for use by the end user.

One can consider also adding a copy method to the Row and Column classes.

Desired solution

Implement _copy() for Table and TaggedTable. Find occurrences of manual copying and replace to use the new method.

[lars-reimann]

Regarding the name: I'd vote for _copy.

[patrikguempel]

used a copy dummy in table#remove_row and table#remove_rows