Generating points for Grid instances must be unique column-wise.

Generating points for Grid instances must be unique columnwise.

The array of generating points provided or created by a generating function for an instance of Grid must now have unique values per column. This is due to each column of the generating points corresponds to the one-dimensional interpolation points in that dimension.

Additionally:

• An array-related utility function is_unique() is added to check if a one-dimensional array has unique values.
• The test suite has been updated to verify these changes.

This commit should resolve Issue #152 (closed) and part of the refactoring of the Grid class (Issue #135).