Refactor class structure

[sklam]

Refactor for the class restructuring suggestion in #38.

• The new Column class corresponds to the Data class in the suggestion.
• GenericIndex now contains Column instead of Series.
• Column contains data and mask. It implements operation on the container (e.g. append). It does not contain operations that requires logical interpretation of the data type, which is done by subclasses of TypedColumnBase:
  • NumericalColumn implements the operations for numeric data.
  • CategoricalColumn implements the operations for categorical data.

Also includes a lot of clean ups of obsolete code due to new implementation in libgdf:

• ad-hoc reduction code is removed
• ad-hoc radixsort code is removed
• unique_k now reuse components shared with groupby.