Add single column select

[anthonymorast]

For example, datatable::DataTable.col() which returns a single column datatable (no response variable) so these can be chained:

datatable::DataTable.col("colname").max()
datatable::DataTable.col("colname").min()

etc.

[anthonymorast]

Will need a min/max with the column specified and another without that only works on single column tables

[anthonymorast]

e.g.
// these two check for single column, error if column count > 0 "column is required for DataTables with multple columns"
T min();
T max();

T min(int col);
T max(int col);

[anthonymorast]

Single column selection was implemented via:

DataTable<T> operator[](std::string column) const;  // select column into DT

Example for min/max chaining:

datatable::DataTable<float> dt(...);
cout << dt["col1"].max() << endl;
cout << dt["col1"].min() << endl;