Inferred schema should know which columns have missing values

[lars-reimann]

Is your feature request related to a problem? Please describe

The inferred schema of a Table currently treat columns of type X with and without missing values the same. For example, in the following table both columns would get type int:

a	b
1	1
2

Desired solution

1. There is already a class OptionalColumnType but it's not used. It should be used to clearly mark columns with missing values.
2. Add an abstract method is_missing or may_be_missing to ColumnType.
3. Implement it for the subclasses.
4. Use this method to reimplement Table.list_columns_with_missing_values and Column.has_missing_values.

[zzril]

Regarding the name of the abstract method, we decided for may_be_missing.

[PhilipGutberlet]

How should the may_be_missing method be used in Table.list_columns_with_missing_values and Column.has_missing_values? If we should use the Column type to clarify if values are missing, the Column type needs to change at the moment were missing values are added to the column.