Remove Generating Values as a Property of Grid

The property generating_values of a Grid instance, as discussed in Issue #71 (closed), is not used anywhere throughout the codebase once the Grid is constructed. It is used as the basis of constructing an array of generating points (which then used in various places instead of the generating values) under the assumption that the generating values apply the same to all dimensions. In case of one-dimensional grid, the first column of the generating points is the generating values.

This property can therefore be safely removed from the Grid class. It will also be removed from the default constructor.

The factory method from_value_set() is kept for the time being; the method is used to create a Grid instance with one-dimensional array that will be expanded according to the dimension of the grid to become an array of generating points.

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