DataTable class enhancements

[akielaries]

The early stage implementation of the DataTable class could use a few short term improvements.

TODO:

• make use of the ThreadPool class within the DataTable methods
• a way to specify columns to convert from DataTableStr -> DataTableInt OR DataTableDouble using the processing functions str_to_int or str_to_double
• template or innate conversion functions
• encoding functions to convert string categorical data to its exact format
  • begs question of homogenous vs heterogeneous data
• UNIT TESTS
• NOTE: when specifying columns, they must be in the order they appear in the file
• a way to drop columns from the DataTable
• a way to