Verify that Each Column of the Generating Points Array has Unique Values

An array of generating points in the Grid instance contains the one-dimensional interpolation points column-wise. Therefore, the values per column must be unique without repetitions.

Because generating points are either generated on the fly by a given generating function or explicitly provided, the uniqueness of the values per column must be establish during the construction of an instance of Grid.

This issue is part of the refactoring of the Grid class (Issue #135).